Which external storage?

By now, you may be more than a little confused over which external storage you should buy for your Mac. Does APFS still work reliably on hard disks? Will the extra cost of USB4 bring faster speeds? What does USB 3.2 Gen 2×2 mean, and do my Mac’s ports support it? Let me try to make the choices a bit clearer.

Internal or external?

One of the most common reasons for buying external storage is to reduce the initial cost of internal storage. Apple doesn’t discount the additional price of larger internal SSDs, and you might think that you can pay for a faster chip option or more memory from the savings you’ll make with a smaller internal SSD. This is a false economy, as:

  • over time, your storage requirements will only increase, and that internal SSD will become fuller;
  • smaller internal SSDs are written to just as often, so they wear out more quickly;
  • internal SSDs almost invariably perform better than the most expensive external SSDs;
  • internal SSDs are far less likely to undergo thermal throttling, because they benefit from your Mac’s internal cooling;
  • larger SSDs perform better than smaller ones.

While huge music collections, Photos libraries and other bulk data are usually best kept on external storage, you should aim to have ample space for everything else on your Mac’s internal SSD for its whole lifetime.

Hard disk or SSD?

Unless you’re still running macOS well before Mojave, you should aim to use Apple’s new file system, APFS, on your external storage. One exception to this is if you’re staying with Catalina or earlier and make Time Machine backups, which still require the older file system HFS+.

APFS is designed for SSDs, and in general use doesn’t work well on hard disks. The most extreme example is a startup disk: if you’ve ever tried booting Mojave or any more recent macOS from a hard disk, you’ll understand only too well. There are, though, well-proven uses for external hard disks with APFS, including:

  • Time Machine backups in Big Sur and later.
  • Static storage of files which change little over time, which can include media libraries.

USB or Thunderbolt?

You may have noticed that USB standards are a bit of a mess, and tricky even for experts to negotiate. By comparison, Thunderbolt is far clearer: for all modern Macs with Thunderbolt 3 or 4 ports, all certified Thunderbolt 3 or 4 devices used with certified cables should deliver consistently high transfer speeds.

Choosing USB devices isn’t easy. It’s not unusual for different users and Macs to see quite different transfer speeds when using what appear to be very similar (or identical) devices. There’s also compelling evidence that many Mac users don’t get the performance that they had expected. However, USB standards are more popular than Thunderbolt, as a result of which you’ll normally pay a premium for the same storage with a Thunderbolt interface.

SSDs come in two broad types: those with a SATA interface, and NVMe.

SATA devices are cheaper and are normally limited to a maximum of 6 Gb/s, which means they’re unlikely to transfer files at more than 500 MB/s, and in some circumstances may operate at 400 MB/s or even slower. This won’t make much difference to Time Machine backups, whose speeds are limited by macOS anyway. They should comply with the USB 3.1 Gen 2 standard or 3.2, making them relatively cheap and available.

If you do need faster transfer speeds, it’s safer to use NVMe over Thunderbolt 3, although there are USB 3.x and USB4 standards which can deliver better performance than you’ll ever get over 3.1 Gen 2. While your Mac might get more than 2 GB/s from one of those drives, it could instead deliver less than 1 GB/s if you’re unlucky. Manufacturer’s claims often prove unreliable, and the only way to find out is to perform your own tests with the Mac you’re going to use.

One way you may be able to get best performance from USB 3.x storage is to connect it to a Thunderbolt hub or dock. If you’re using a notebook, this helps solve port problems as well as making consistent performance more likely than by connecting the storage direct to a port on your Mac.

One last but important consideration is the ability to check and monitor storage health: macOS and third-party tools like DriveDx can access S.M.A.R.T. indicators on devices connected using Thunderbolt. But to do the same for those using any form of USB, including USB-C, requires installation of a kernel extension. That can cause problems with M1 Macs, as it requires their security to be downgraded, while Thunderbolt storage doesn’t require the kext or downgraded security.

Cooled or compact?

SSDs generate heat, and keeping them cool is an important consideration. Those mounted inside your Mac are designed into its cooling system, which is just as well because, being so fast, they can generate a lot of heat. Compact external storage only allows for passive cooling methods; put a high-speed NVMe module in it and it can quickly become so hot that its operation has to be slowed by thermal throttling. This is most important if your Mac writes large amounts of data continuously to the SSD, when throttling can reduce its performance to the same as that obtained in much cheaper storage.

External enclosures with fans are expensive, but if you want to ensure that an SSD’s write performance remains as good as claimed by its manufacturer, you’re likely to find them much better than any compact case. This shouldn’t affect cheaper and slower SATA SSDs, though.

Branded or separates?

Buying a complete storage unit with a reputable brand name on it may seem safer than putting modules intended for internal use inside a separate enclosure or case. However, branded compact units also have disadvantages, in that you can’t move the SSD inside to a different enclosure/case, and you normally don’t get to specify the model of SSD used inside it.

External boot disks

There was a time when you could carry a portable disk in your pocket and boot any of your Macs, and those of other people, from that disk. While that’s still largely possible, even with T2 and M1 models, it’s far less attractive now, and unlikely to be a good plan for the future. Secure Boot for T2 Macs assumes that they’re run in their default configuration, which blocks them from booting from an external disk. While M1 Macs have been designed to enable that, it’s more complex than booting from the internal SSD, and consequently less reliable.

Booting Big Sur and later expects high performance from the boot disk, which is readily achieved from an internal SSD but requires considerably more costly external SSDs. If you need to boot from an external SSD occasionally, this shouldn’t be a problem. For example, if you need to run several different versions of macOS for testing purposes, an external SSD can be an excellent solution. However, it’s unlikely to prove a good solution in regular and normal use.

Return and refund

Whatever you choose, ensure that you can return the product for a full refund. When you first get it, check its performance carefully with the Mac you’re intending to use it with. If you’re unhappy, and can’t get the transfer rates you need, don’t hesitate to send it back and pick a replacement which does the job.