Dr. Mac’s Rants & Raves
Episode #393

I’m frequently asked for advice on adding storage to a Mac, so I distilled my thoughts into a checklist to help you decide on your next storage upgrade.

√ How much storage do you need?

If you’re going to use the drive for backups using Time Machine or other software, it’s best to get a disk at least 3X the size of your boot drive.

So, if your Mac has a 512GB solid-state drive (SSD), you want at least a 1.5TB drive, though 2TB would be better (and probably more cost-effective).

√ How will you use the disk?

If you intend to boot your Mac from this disk, you will be happier with an SSD. macOS is optimized to run on an SSD. Booting from a hard drive takes much longer than from an SSD, and almost everything — launching apps, opening applications, opening windows with more than a few files—will feel sluggish.

On the other hand, if you plan to use it for backups or additional storage for your music, photos, or videos, a hard drive will give you more storage for less money. Which brings us to…

√ What is your budget?

Solid-state drives are much faster, but also more expensive than hard drives. For example, a 1TB external USB 3 SSD will cost you around $160, while a 1TB USB 3 external hard drive is approximately $50.

The higher the capacity, the greater price difference.

So, opt for solid-state if you can afford it—it’s way, way faster. But, unless you intend to boot from the drive (or have another reason for needing SSD speeds), a hard drive delivers more bang for your buck.

√ Internal or external?

This one’s easy—unless you’re replacing an internal drive because it died or is too small—an external drive doesn’t cost much a lot more than an internal one, and you (or a service technician) avoid poking around your Mac’s guts.

√ What ports are available on your Mac?

You want to use a port with USB 3.0 or higher in most cases. If your Mac has an available Thunderbolt port, Thunderbolt drives are rarer but can be significantly faster than a USB 3.0 drive. But… Thunderbolt drives are considerably more expensive than USB 3 drives.. So, unless you truly need super-fast storage, USB 3 is going to be your best bet.

√ What brand do you recommend?

None. I buy the least expensive drive that meets my needs. Since all drives fail eventually, I just assume my drives will fail tomorrow and maintain redundant backups of every file on every disk.

√ One More Thing: You might want to consider something like this Voyager Dock if you expect to have more than two external disks:

A Voyager Dock like these let you use less expensive "bare" 2.5 or 3.5-inch hard disks.

A Voyager Dock lets you pop-in less-expensive “bare” 3.5-inch (left) or 2.5-inch (right) hard drives.

The Voyager Dock lets me buy less expensive “bare” (aka “internal”) 2.5 or 3.5-inch hard disks, which have no enclosure or power supply so they take up less space while cost less per gigabyte.than separate external drives..

I have more than half a dozen backup disks, so I have a dock with two bays that allows me to mount two disks at a time (instead of just one):

OWC USB-C Dual Drive Dock ($89.75).

OWC USB-C Dual Drive Dock ($89.75).

Yes, I still have to swap disks by hand, but that’s a small price to pay for saving me a lot of desk space, needs only one USB port, and cost a lot less than 6 separate external USB drives with enclosures and power supplies.

Trust me: If you need (or think you’ll someday need) two or more additional external disks, you can’t go wrong with either a Single Bay Dock ($34.75) or Dual Bay Dock ($89.75) from Other World Computing.

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Thanks for the wise advices. Comments based on our experience at the University with thousands for Macs over decades: 1. Once you try SSD, you do not want mechanical rotational disk (HDD), even for free. 2. Not all disks were created equal. That includes SSD. Some of them work fine, but others may generate all kinds of issues on Mac: unexpected ejections, freezings, crashings, thermal throttling and even kernel panics. Sometimes, the issues are due to cables, supplied by the manufacturer (!!!), and others to internal electronics. Sometimes, such issues are not revealed in published reviews, so in-house testing becomes… Read more »

Lee Dronick

Thank for that info.


Thanks for the info about DriveDx–I’ve been looking for something like that. The device driver it requires for external drives makes me a little nervous but is necessary for the current Mac OS design. As for SSDs, I recently bought a Sandisk Extreme Pro 1TB SSD that I really like because it is the fastest external device I have ever used! It’s a bit pricey but worth it if you need speed (and not to be confused with the non-Pro version that is slower).


Thank you in the first place for bringing this interesting topic to our attention. You know, this is what makes Internet so comprehensive and useful. Keep up the great work!


I tried to further edit my message above, but the timing has expired. It would be great if that could be extended or removed. Just a suggestion for your consideration. So here it goes: Thank you in the first place for bringing this interesting topic (which is not always fully understood) to our attention. You know, this is what makes Internet so comprehensive and useful. Keep up the great work! And there is more… 15. In relation to topic 11 above, another jewel is DiskWarrior, which can repair disks (HFS+ now; hopefully APFS also in the near future) when other… Read more »

Last edited 9 days ago by Macsee

20. And finally, it is recommended to make sure that the Mac booting disk has at least 10% free space. That is because macOS needs free space to do housekeeping activities. Otherwise, it may cause data corruption and loss. The consequences could be catastrophic. That does not apply to other non-booting disks, which can be filled (for instance, to store movies). When in need of a larger disk, a good rule of thumb is to purchase one that has twice capacity than the current one to be replaced. Purchasing much larger ones may be a waste of money, since technology… Read more »