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Fusion drive as storage?  



Yesterday's lack of a new iMac announcement has me looking anew at my upgrade options. I’m <still!> running a 2007 iMac in my home office. It’s got an SSD, and it’s served me well. I setup a 2017 3.0ghz/16gb/1tb HDD iMac recently for a friend, and was shocked at how slow it seemed in comparison to my 11 year old machine. That hard drive is amazingly poky.
I had hoped that Apple would start shipping iMacs with a less expensive SSD option ($700 for 1tb, really?! And only 500gb available for 21.7” machines), or do away with the HDD option altogether, but no such luck. I’ve installed SSDs in plenty of machines, but the newer iMacs give me pause.
In lieu of a built in SSD, I purchased a (used) LaCie Rugged 2tb Thunderbolt/USB-C HDD, took the drive out, and replaced it with a 1tb Crucial M500 1tb SSD. My plan is to use this as the startup drive via USB-C/Thunderbolt 3.
So, with all that laid out, my actual question is, does it make any difference whether I order the model with an internal HDD or a fusion drive? If I use the internal drive as storage, is there any speed advantage to having a fusion drive? Or does it only take advantage of its onboard cache when used as a startup drive? I could also use the internal drive as my backup clone, in which case speed only matters if my external SSD fails.

This topic was modified 2 years ago by blocktek
6 Answers

I have only ever used a Fusion Drive as the startup disk, but given that you can "build" one via Terminal commands I cannot see any reason why it would not work as a non-startup disk.


Old UNIX Guy


Agree with @kevinbuterbaugh — this should work fine, and I think your plan is a smart one, @blocktek

But will there be any speed advantage in this configuration? At least enough to justify the extra cost?


I'd say the answer to that is going to heavily depend on what you're using the storage for.  Again, I've never used a Fusion Drive for anything other than a startup volume.  I can tell you that in that scenario it does help significantly.  Of course it's not as good as a pure SSD.


I had a mid-2011 27" iMac that formerly had only an internal HD.  I installed an SSD in it (behind the DVD drive) and turned the SSD and HD into a Fusion Drive via Terminal commands.  I can tell you that for that configuration it cut my boot times in half.


I know a lot of people are ambivalent about Fusion Drives, or maybe even a little scared about using them, but I think that while they're not nearly as good as pure SSD's, they're also a ton better than just pure HD's...


Old UNIX Guy


Could I ask Old UNIX Guy the following: I have a new 2019 4K Mojave iMac with a 1TB internal HDD (I naively thought it was SSD, but that's another matter...). I presume that it is an 'old school' spinning HDD (as I can not seem to find any more system details). I already intended to install a 1TB  SSD external drive since, as a keen photographer, I always need more storage space and processing speed.

My question: Can I (and is it sensible) to configure the external SSD as the everyday 'boot drive' (and transfer all applications there) whilst using the internal HDD as my photo-storage? If so, I have also seen remarks about needing to set Terminal commands (but what and why....?) since Mojave has a setting in Preferences asking from which drive the system should boot.

If I make this switch what would be the process? Thanks in advance!

This post was modified 1 year ago by PeterMac

Hi PeterMac,

Yep, you sure can. That's actually what I do. You'll need to hook up the SSD and do an install to it. Then boot off of it and use System Preferences to set it to be the boot drive. After that, I'd test booting off of it and then reformat your internal HD with Disk Utility. To have your Photos on the internal drive you can set that in Photos preferences I believe ... but I'm, well, the Old UNIX Guy, so I just create a symbolic link from /Users//Photos to the location on the internal drive.

Old Unix Guy

Thanks Old Unix Guy, that looks like a good plan. Thanks for your help.

For my ignorance, Old Unix Guy, can I just ask re your comment: "reformat your internal HD with Disk Utility" - presumably this is to ensure the current OS on the internal disk is fully removed and so you are left with a pristine HDD formatted like 'new'.

However, as I now have all my user files and photos on the internal HDD, I guess what I really want to do is just 'remove' the old internal OS (when the new external SDD is fully checked and booting sweetly) and leave the user files and photos in place. Is there a tidy and 'correct' way of doing that?

Ideally I would also like to have all my currently installed applications running from the SDD (for loading speed etc). Is there a neat and tidy way to 'clone' these to the new SSD?

Sorry for asking such a lot of questions of you!


I have a 2011 iMac which has an internal 500GB HD. It started failing so I have attached an external 1TB via Thunderbolt. I have have set up /etc/fstab so internal disk doesn’t mount. Boot time has improvise massively but so has swapping between apps. 

Of course if there hadn’t been anything wrong with internal HD I could have had it mount and used as extra storage.

Remember to include the internal HD in your backups.

This post was modified 1 year ago by Stephen Bedford

The other option is to not use the internal HD, being the bit that’s most prone to wear. Spin it up once a month or so but rest of time have it dismounted. That extends the potential lifetime for next user of machine,

This post was modified 1 year ago by Stephen Bedford

Thanks Stephen, that is really useful thinking. Thanks also for your help.

I'd test booting off of it and then reformat your internal HD with Disk Utility. To have your Photos on the internal drive you can set that in Photos preferences I believe