The same happened with good old USB-A: even when your Mac has four, you quickly accumulate more USB-C devices than you’ve got ports. Yet no one seems to be offering a ‘port expander’ to turn a single USB-C port into several. In particular, I’m now accumulating several external SSDs in compact cases. One obvious step forward would be to mount two or more in a single housing, which is the aim of this article.

Apart from my Samsung X5 SSD, all my external SSDs are of the ‘slow’ SATA type, so I wasn’t in the very up market for an expensive Thunderbolt 3 RAID system, yet. Maybe that’s what I’ll replace my existing Promise Pegasus T4 RAID system with when it finally dies, or when Time Machine 2.0 ships, whichever is the sooner. But for the moment, a reliable USB 3.1 housing for two or more SATA SSDs will do me nicely, thanks.

This appears to be a new market, currently satisfied in the main by a range of East Asian manufacturers whose names I don’t know. Whether bought from a specialist storage dealer or retailer like Amazon, the closest I could find to a recognisable name with effective US or European representation was StarTech’s S252BU313R USB 3.1 External Enclosure for Dual 2.5 Inch drives. At slightly less than $/€/£ 100, its cost equates to around 1 TB of SSD storage, which is far better than that of an unpopulated Thunderbolt 3 SSD RAID array.

This enclosure is well engineered, with a neat black metal case which is around the size of four compact external SSD cases stacked on top of one another. Each drive is installed in a plastic sled, with four small crosshead screws to retain it in place. The sled is then pushed along the runners inside the case for its connectors at the far end to mate securely. Finally, the face of the case is secured using four small Allen key bolts. That took just a few minutes.

Another advantage of this housing for desktop systems is that it’s not powered over USB-C, but from a neat mains adaptor. So it has a small fan, which seems fairly puny, but it’s essentially silent, and manually switchable from off to 100% on the rear of the enclosure. As these are SATA SSDs, the fan is unlikely to be needed: I’ve never noticed a compact external SATA SSD get in the slightest bit warm, unlike my Samsung X5 which gets quite warm even when sitting idle.

StarTech provides two cables, one which takes the Micro B socket on the back of the enclosure to USB-C, the other to USB-A. Best of all, the enclosure has lights! On its face are three LEDs, which show blue for power and each of the two SSDs, with the latter flashing red when transferring data.

The enclosure provides four hardware configurations for the drives: JBOD, which treats them as two separate disks, RAID 0 (striping), RAID 1 (mirroring), and SPAN, which turns the two into a single virtual drive. This is set not by a driver or utility which could cause support and compatibility issues, but with a pair of small switches on the back of the case.

I opted for JBOD, as one of the 1 TB SSDs had previously been the external SSD with the greater part of my Home folder on it, which I wanted to keep intact. I ensured that I not only had a Time Machine backup of that, but also made a copy of much of it to my Samsung X5 before shutting down, installing both disks in the enclosure, and trying it out.

I connected the enclosure to one of the USB-C ports on my iMac Pro, powered the enclosure and my external RAID up, and once all lights were blue, started the Mac up. Because there’s no software configuration for this enclosure, everything just worked. I formatted the new SSD inside it, and remembered to set Time Machine to back up that disk as well, as by default new disks get added to the exclude list in the Time Machine pane.

I used a Finder Alias rather than symbolic link from the old SSD to my main Home folder on my iMac Pro’s internal SSD. Because the name of the existing AFPS volume hadn’t changed when it was moved from its old external case to the new StarTech enclosure, macOS didn’t bat an eyelid and the alias worked fine, as would a symlink of course. If you reformat the disks to run as a RAID pair or spanned into a single virtual disk, you need to ensure that the volume name remains the same for previous aliases/links to continue to work.

Tested using Blackmagic Disk Speed Test, the two SSDs in the StarTech enclosure deliver almost identical performance to other similar SATA SSDs running over USB 3.1: they read at around 530 MB/s, and write at 470-500 MB/s. I’ve now doubled my external working storage without losing another USB-C port, and without the expense of a 2 TB SSD.

Had the StarTech enclosure not offered its RAID and SPAN options, configuring the two drives as one would have been more tricky. Unlike some file systems, such as ZFS, APFS isn’t designed to span physical disks. AppleRAID, as offered by RAID sets in Disk Utility, has some significant limitations, notably the fact that you can’t boot from a RAID set. However, in addition to RAID levels 0 and 1, it does offer the option to concatenate physical disks into one. My preferred software option would have been SoftRAID, but it neither supports spanning nor APFS, although the latter is coming soon in version 6.

So for the moment, unless you use hardware support for spanning or RAID 0, as offered in this StarTech enclosure, a multi-disk system will deliver multiple volumes rather than using the drives together as a single disk, which seems surprisingly primitive for a mature operating system like macOS.