How much is that external SSD cable?

If you have a Mac with Thunderbolt 3 ports and any external SSDs, you may be wondering how best to connect them. The choice has been limited in the past: Thunderbolt drive enclosures were hard to obtain and costly. When you did fork out a premium price for one, its internal electronics were unlikely to deliver much better performance than USB3.

My benchmarks are taken from earlier tests which I ran on a similar SSD in USB3 and Thunderbolt 1 enclosures: Thunderbolt delivered 345 MB/s writing, and 352 MB/s reading; USB3 was slightly slower at 280 MB/s writing, and 339 MB/s reading. For comparison, a Thunderbolt 1 hardware RAID array on hard drives achieved 377 MB/s writing and 273 MB/s reading.

I mounted a Crucial CT1000MX500 1 TB 3D NAND SSD with a 6 Gb/s specification in an Ineo USB C 3.1 Gen 2 external case costing just under £20, with claimed 10 Gb/s performance. This I connected to the upper (left) Thunderbolt/USB-C port on a MacBook Pro 14,1 running High Sierra 10.13.5. The SSD was freshly formatted in APFS with no files stored on it.

I then ran the 5 GB stress tests in Blackmagic Disk Speed Test 3.1 with a variety of cables connecting the external SSD to the MacBook Pro.

The cables that I tested were:

  • A Belkin Thunderbolt 3 USB-C to USB-C Cable, 50 cm long, costing £25
  • The ‘special’ Thunderbolt 3 USB-C to USB-C cable provided with my CalDigit TS3 dock, 50 cm
  • Two free USB-C to USB-C cables provided with Ineo external cases, 31 cm
  • A red Anker PowerLine+ USB-C to USB-C cable, 180 cm, costing £11.

Results from these were essentially identical, apart from the long Anker PowerLine+ cable. All the others attained just over 450 MB/s writing, and just over 500 MB/s reading. The Anker PowerLine+ cable was much slower, at just over 40 MB/s writing and reading. For comparison, the MacBook Pro’s internal SSD was far quicker, at around 1900 MB/s writing, and almost 2500 MB/s reading.

It’s worth considering the claimed maximum performance of each link in the chain. My MacBook Pro’s Thunderbolt 3 ports have a maximum throughput of 40 Gb/s, which is 5000 MB/s. The drive enclosure claims to be capable of 10 Gb/s, which is 1250 MB/s. However, the SSD itself is specified as 6 Gb/s, which is 750 MB/s, and – ignoring the cable – the slowest part of the process.

Read speeds of just over 500 MB/s are getting close to the SSD’s practical maximum, and write speeds were not far below that.

The only cable which didn’t realise such good performance was the Anker, which is intended for charging and general purposes, and not for connecting SSDs. It was also much longer than the other cables, by more than a metre.

To achieve significantly higher speeds, comparable with those of the internal SSD, would require expensive hardware, such as a hardware RAID array of SSDs with a full-performance Thunderbolt 3 port. That might justify the high cost – over £50 in some cases – of claimed ‘high performance’ Thunderbolt 3 cables.

As it is, I have a fine 1 TB SSD for the cost of the disk itself plus case and cable for under £20. That brings the total cost to less than £220, compared with ready-made equivalents costing between £255 and £315. I also note that the most expensive of those ready-made drives is claimed to deliver “up to 550 MB/s read”, which is about where mine is too.

Paying more doesn’t always get you better.