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[Solved] Starting Up from External SSD, Ignoring internal Fusion Drive


Dan Wieder
(@dwieder)
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I recently purchased an external SSD to use as my startup disk for my 2015 iMac with a Fusion Drive. Now when I boot up, a dialog box always comes up asking to unlock the internal disk. I always hit cancel. Does anyone know if there is a way so this dialog box doesn't come up at all?

Screen Shot 2021 07 29 at 9.12.28 AM

 


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cpragman
(@cpragman)
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The OS default behavior is to attempt to mount all connected drives upon system startup.  You can override this behavior by creating the config file /etc/fstab.  On my system, I have /etc/fstab set to not mount several external disks I keep around for troubleshooting, occasional system images, etc.  By not having those disks mounted, the computer doesn't spend any time indexing their contents, and they are less likely to get corrupted since they aren't being accessed.

 

To prevent a specific volume from auto-mounting, you first need to know its UUID.  You can obtain this from Disk Utility (use the "Info" button when the drive is selected).  Click on the UUID and you can copy it to your clipboard.

 

In my example here, I have a volume named "Untitled" that I don't want to mount at boot time.  Its UUID is 3C24D181-A1A8-4EF6-80FD-00A5F52F0491

Screen Shot 2021 07 31 at July 31, 2021 09.24.18 AM

 

Next you want to create the /etc/fstab file.  I do this via terminal and the vi editor, but other plain text editors also work (ex., BBEdit).  You need to use sudo to create/edit the file, because it's owned by root.  For example:

$ sudo vi /etc/fstab

 

You add one line to the fstab file for every drive you don't want to mount.  The "noauto" argument is what prevents auto-mounting.  It's important that there be no space between the comma and "noauto" or it won't work.  The other arguments are the UUID you copied from Disk Utility, and the parameters that match the way the drive is formatted (i.e., apfs or hfs).  If the drive was HFS+ you would put "hfs" instead of "apfs" where you see it below.

So on my machine, the fstab entry to prevent the drive Untitled from mounting is as follows:

# prevent External Untitled 

UUID=3C24D181-A1A8-4EF6-80FD-00A5F52F0491 none apfs rw,noauto

 

Note that even though the drive no longer auto mounts at boot time.  It can still be manually mounted when you need it via Disk Utility.  

I have a few bootable external volumes I don't want mounting.  For recent versions of MacOS, the boot disk is split into two volumes.  One is for the system, and the other for user data.  (ex., "Macintosh HD" and "Macintosh HD - Data")  Just to be sure, I added the UUIDs for both of these volumes to my fstab file.


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Dan Wieder
(@dwieder)
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@cpragman 

Well thanks for the information, but I must still have something wrong. I got the UUID for both the Macintosh HD and Macintosh HD - Data and created the following /etc/fstab:

[imac:~] becky% more /etc/fstab

# prevent Internal Macintosh HD UUID=319BF3B7-3CFC-41A9-8225-7D03C0057CBD none apfs rw,noauto

# prevent Internal Macintosh HD - Data UUID=116177ED-05AA-4C7B-A725-44DC8359F93F none apfs rw,noauto

However, when I rebooted I still get the dialog boxes asking to unlock both disks.

Is it possible that I have to unlock them first? Does having Filevault enabled affect this?


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cpragman
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Yeah. FileVault adds another level of abstraction (container inside a container) each level of which has a different UUID. Try unlocking the disk and then see if it offer you different UUIDs. It won’t hurt to just add more UUIDs to fstab until you find the ones that actually work. 

you might also want to try the following terminal command to see a lot more detail about all the levels of all the containers. 

$ diskutil apfs list


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Dan Wieder
(@dwieder)
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@cpragman 

So that didn't do it. The UUID doesn't change after I mount them.

 

I see the following from the diskutil command:

|   +-> Volume disk2s1 116177ED-05AA-4C7B-A725-44DC8359F93F

|   |   ---------------------------------------------------

|   |   APFS Volume Disk (Role):   disk2s1 (Data)

|   |   Name:                      Macintosh HD - Data (Case-insensitive)

|   |   Mount Point:               Not Mounted

|   |   Capacity Consumed:         475731906560 B (475.7 GB)

|   |   Sealed:                    No

|   |   FileVault:                 Yes (Locked)

 

Within Disk Utility I see which goes along with the diskutil command.

Macintosh HD - Data

APFS Data Volume • APFS (Encrypted)

 

Do file permissions get involved on the /etc/fstab?

[imac:~] becky% ls -l /etc/fstab

-rw-r--r--  1 root  wheel  195 Jul 31 08:13 /etc/fstab

 

The disk was under Catalina when FileVault was used to encrypt it. Is it possible I need to somehow turn FileVault off first?


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cpragman
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Unlock the disk, then run

$diskutil apfs list 

again


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cpragman
(@cpragman)
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Here on my laptop which has FileVault turned on for the boot drive, the result looks like this.  Note this system is Catalina.

$  diskutil apfs list
APFS Containers (2 found)
|
+-- Container disk1 C9918A4C-5DCA-4DFB-97BD-FB31F4ACBDB8
| ====================================================
| APFS Container Reference: disk1
| Size (Capacity Ceiling): 1890173652992 B (1.9 TB)
| Capacity In Use By Volumes: 802580127744 B (802.6 GB) (42.5% used)
| Capacity Not Allocated: 1087593525248 B (1.1 TB) (57.5% free)
| |
| +-< Physical Store disk0s2 45DCE8BD-7082-4609-874A-9CBBC16B1E79
| | -----------------------------------------------------------
| | APFS Physical Store Disk: disk0s2
| | Size: 1890173652992 B (1.9 TB)
| |
| +-> Volume disk1s1 A15E4A3C-1A3B-4D3A-B98C-3C3A3D93B67C
| | ---------------------------------------------------
| | APFS Volume Disk (Role): disk1s1 (Data)
| | Name: Macintosh HD - Data (Case-insensitive)
| | Mount Point: /System/Volumes/Data
| | Capacity Consumed: 786185080832 B (786.2 GB)
| | FileVault: Yes (Unlocked)
| |
| +-> Volume disk1s2 1852BA36-42F0-4002-B530-60AF6884B6E5
| | ---------------------------------------------------
| | APFS Volume Disk (Role): disk1s2 (Preboot)
| | Name: Preboot (Case-insensitive)
| | Mount Point: Not Mounted
| | Capacity Consumed: 27508736 B (27.5 MB)
| | FileVault: No
| |
| +-> Volume disk1s3 CC28404B-BD50-40D9-BB78-940FF3A727A8
| | ---------------------------------------------------
| | APFS Volume Disk (Role): disk1s3 (Recovery)
| | Name: Recovery (Case-insensitive)
| | Mount Point: Not Mounted
| | Capacity Consumed: 525848576 B (525.8 MB)
| | FileVault: No
| |
| +-> Volume disk1s4 19886AFF-9A48-4175-8830-478C02B11E3C
| | ---------------------------------------------------
| | APFS Volume Disk (Role): disk1s4 (VM)
| | Name: VM (Case-insensitive)
| | Mount Point: /private/var/vm
| | Capacity Consumed: 4296044544 B (4.3 GB)
| | FileVault: No
| |
| +-> Volume disk1s5 2B001D43-1D54-4538-BF33-6B58EC6841AE
| ---------------------------------------------------
| APFS Volume Disk (Role): disk1s5 (System)
| Name: Macintosh HD (Case-insensitive)
| Mount Point: /
| Capacity Consumed: 11255320576 B (11.3 GB)
| FileVault: Yes (Unlocked)
|

 

 

Given the results above, I'd add the UUIDs of disk1s1 and disk1s5 to my fstab file.  that is...

UUID=A15E4A3C-1A3B-4D3A-B98C-3C3A3D93B67C

and 

UUID=2B001D43-1D54-4538-BF33-6B58EC6841AE


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Dan Wieder
(@dwieder)
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Posts: 57
Topic starter  

@cpragman 

No luck yet. Still asks me to unlock the disks after I rebooted.

So here is what I get:

APFS Containers (3 found)

|

+-- Container disk2 4B7FF895-F70E-4A53-99B3-CEEDBDB36489

|   ====================================================

|   APFS Container Reference:     disk2 (Fusion)

|   Size (Capacity Ceiling):      1023683072000 B (1.0 TB)

|   Capacity In Use By Volumes:   494214070272 B (494.2 GB) (48.3% used)

|   Capacity Not Allocated:       529469001728 B (529.5 GB) (51.7% free)

|   |

|   +-< Physical Store disk0s2 B6002E6B-DCCE-4CDD-A5EF-7C528AE95AAD

|   |   -----------------------------------------------------------

|   |   APFS Physical Store Disk:   disk0s2 (Main, "Faster" Disk Use)

|   |   Size:                       23687942144 B (23.7 GB)

|   |

|   +-< Physical Store disk1s2 A86E5190-44AA-4D7B-B52E-77952A27186A

|   |   -----------------------------------------------------------

|   |   APFS Physical Store Disk:   disk1s2 (Secondary, Designated Aux Use)

|   |   Size:                       999995129856 B (1000.0 GB)

|   |

|   +-> Volume disk2s1 116177ED-05AA-4C7B-A725-44DC8359F93F

|   |   ---------------------------------------------------

|   |   APFS Volume Disk (Role):   disk2s1 (Data)

|   |   Name:                      Macintosh HD - Data (Case-insensitive)

|   |   Mount Point:               /Volumes/Macintosh HD - Data

|   |   Capacity Consumed:         475724410880 B (475.7 GB)

|   |   Sealed:                    No

|   |   FileVault:                 Yes (Unlocked)

|   |

|   +-> Volume disk2s2 CCA5136B-A79D-45FD-B58A-A6C38588E85A

|   |   ---------------------------------------------------

|   |   APFS Volume Disk (Role):   disk2s2 (Preboot)

|   |   Name:                      Preboot (Case-insensitive)

|   |   Mount Point:               Not Mounted

|   |   Capacity Consumed:         82894848 B (82.9 MB)

|   |   Sealed:                    No

|   |   FileVault:                 No

|   |

|   +-> Volume disk2s3 EB4F7DCF-111E-4962-8A01-0BDFEDE0E731

|   |   ---------------------------------------------------

|   |   APFS Volume Disk (Role):   disk2s3 (Recovery)

|   |   Name:                      Recovery (Case-insensitive)

|   |   Mount Point:               Not Mounted

|   |   Capacity Consumed:         528539648 B (528.5 MB)

|   |   Sealed:                    No

|   |   FileVault:                 No

|   |

|   +-> Volume disk2s4 63C773DC-B1DB-4E99-9933-4B47E446A8FD

|   |   ---------------------------------------------------

|   |   APFS Volume Disk (Role):   disk2s4 (VM)

|   |   Name:                      VM (Case-insensitive)

|   |   Mount Point:               Not Mounted

|   |   Capacity Consumed:         2148556800 B (2.1 GB)

|   |   Sealed:                    No

|   |   FileVault:                 No

|   |

|   +-> Volume disk2s5 319BF3B7-3CFC-41A9-8225-7D03C0057CBD

|       ---------------------------------------------------

|       APFS Volume Disk (Role):   disk2s5 (System)

|       Name:                      Macintosh HD (Case-insensitive)

|       Mount Point:               /Volumes/Macintosh HD

|       Capacity Consumed:         11211423744 B (11.2 GB)

|       Sealed:                    No

|       FileVault:                 Yes (Unlocked)

|

 

My /etc/fstab now looks like:

# prevent Internal Macintosh HD UUID=319BF3B7-3CFC-41A9-8225-7D03C0057CBD none apfs rw,noauto

# prevent Internal Macintosh HD - Data UUID=116177ED-05AA-4C7B-A725-44DC8359F93F none apfs rw,noauto

# prevent Internal Macintosh HD UUID=4B7FF895-F70E-4A53-99B3-CEEDBDB36489 none apfs rw,noauto

# prevent Internal Macintosh HD UUID=B6002E6B-DCCE-4CDD-A5EF-7C528AE95AAD none apfs rw,noauto

# prevent Internal Macintosh HD UUID=A86E5190-44AA-4D7B-B52E-77952A27186A none apfs rw,noauto

# prevent Internal Macintosh HD UUID=CCA5136B-A79D-45FD-B58A-A6C38588E85A none apfs rw,noauto

# prevent Internal Macintosh HD UUID=EB4F7DCF-111E-4962-8A01-0BDFEDE0E731 none apfs rw,noauto

# prevent Internal Macintosh HD UUID=63C773DC-B1DB-4E99-9933-4B47E446A8FD none apfs rw,noauto


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cpragman
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Not sure if it's the forum site here doing something screwy with the fstab you pasted or not.  In the fstab file, lines starting with a pound sign are comments.

You want the lines with the actual UUID= instructions to start on separate lines

 


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Dan Wieder
(@dwieder)
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@cpragman 

Well duhhh! It's been a few years since I used my unix skills. I knew that was a comment and completely forgot about it. Let me fix that and try again. Thanks for the reminder. 🙂


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Dan Wieder
(@dwieder)
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@cpragman 

Worked perfectly!

Thanks for the information and thanks for sticking with me.


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