A Brief Primer on Modern External Storage
Howard Oakley recently had an excellent article on this topic (see above) and I realized things have changed quite a bit for us Mac users since TB4 first came out. External storage is still “slow” compared to what a modern M3 Mac achieves internally (~ 7 GB/s), but it has become a lot better. Time to revisit a couple of things, so here goes. Note this is primarily intended for Apple Silicon Macs equipped with TB4 ports that support 10 Gbps USB3 or more recent Intel Macs with TB3 ports that support 10 Gbps USB3. If you have only 5 Gbps USB3 ports or TB2, you don’t have any fast storage at all. Get a new Mac first. If you cannot afford ordering it with lots of its very fast internal flash (“SSD”), read below for what to look out for when purchasing external flash.
First things first: get flash and enclosure separately. That way you know exactly what you are getting, you can exchange either part if it does not perform up to spec, and in the case that you need to replace one, you replace only what actually needs replacing. Plus you do not pay the markup that manufacturers charge for the “convenience” of packaging a solution for you.
If you are interested in fastest external SSDs, you first need to be getting the fastest available enclosure. Presently that means getting an NVMe enclosure with the ASM2464PD/PDX controller from ASMedia. This is the fastest controller on the market and it has support for PCIe Gen4x4 (4 PCIe lanes at their full b/w) which in reality translates to about 3.2+ GB/s (if you use quality flash). It actually allows tunneling the full PCIe b/w to the external client rather than only a portion of it due to fixed reserves for DP overhead like some earlier controllers (“TB3”). Most TB controllers you find are Intel JHL7440 (Titan Ridge) which support PCIe Gen3x4 (24 Gbps, limited by the controller rather than offering full 4x 8 Gbps) which in reality at very best will get you 2.8 GB/s. This is why for the longest time people have warned that although TB3/4 claims to offer 40 Gbps, the best data transfer you’ll see is about 2800 MB/s.
Getting an enclosure with ASM2464PD/PDX is straightforward and there are many (here’s an inexpensive [$80] example, there are even some with little built-in fans [$90], many more listed here). The only issue here is that if you connect that “USB4” enclosure to a non-TB USB-C port, what will that enclosure fall back to? Most would expect 10 Gbps USB 3.2 gen 2 or similar (Macs in general do not offer support for 20 Gbps USB 3.2 gen 2x2), but that’s not necessarily a given (see aforementioned link). If you’re connecting to a Mac’s TB3 or TB4 ports, that’s not an issue though.
Once you have a really fast enclosure, you need to make sure that the NVMe flash you put into it, is actually fast enough to saturate the ~3.2 GB/s offered by your enclosure and the TB bus. A good example for that — and always assuming 2 TB capacity for these examples — is the $170 Samsung 980 PRO that has been shown to achieve excellent performance when used externally with TB on Macs. Note it also routinely achieves much better throughput (at least on Macs) compared to the 970 PRO. Other good options are $157 WD Black or $183 Crucial P5 Plus. The usual buzzwords here are SLC over TLC over QLC, DRAM, etc. But you can ignore all that if you stick to one of these three. With one of these you’ll be fine. There are others that are perhaps cheaper, but good luck getting exact specs on the employed flash and controllers. (not saying all others stink, it’s just difficult to get good hard specs)
Now, if you are not out for best performance, chances are you’re looking for either media storage or backup. This is important because you can indeed save a lot of money if you use 10 Gbps USB3 instead of TB for that purpose. You can get a decent enclosure for below $20 (like this Orico for $15) and the flash is also much less expensive because now all you need is something that pushes 1 GB/s rather than the higher rates mentioned above. An example for this is the Intel 670p (often sold out) or the well-rated Silicon Power UD85 at just $108 for 2 TB.
Finally, a word on Time Machine storage. You likely still want an SSD because HDDs are mechanical, break down, make noise, and draw more power. But you don’t want a fast SSD. Reason for this is that TM backups are i/o throttled by macOS on purpose. You will never see a TM backup at over 300 MB/s sustained (or burst for that matter). And yes, that includes the initial TM backup where you’ll be transferring dozens or even hundreds of GBs and would like to see it go faster. But macOS doesn’t provide for that. So that means you’re essentially looking for ~3 Gbps SSDs. Does that mean you should get a SATA SSD? No! Although SATA’s peak b/w of 6 Gbps is more than enough, SATA SSDs these days are more rare and hence usually (much) more expensive than the more widely available NVMe flash. But this TM throttling does mean that you could focus on 10 Gbps USB3 enclosures and slower flash if all you’ll be doing is TM backup. That equates to a lot of savings with only 2 real downsides:
- SMART monitoring on Macs works just fine for TB-attached storage, but not for USB3-attached storage. TRIM on the other hand should also be available over USB3 as long as you’re sticking to NVMe over SATA.
- If you want to copy data from that TM drive to a Mac over TB but through eg. Finder, you won’t be subjected to TM throttling. Your call if you think that will happen frequently enough to justify the extra expense.
Final word of warning. If you want fast external flash and you have an Intel Mac, keep in mind to plug into the right ports. Intel controllers have two ports on one controller and they have to share b/w. If you have more than one fast external flash hooked up to your Mac at a time, make sure to choose two ports that are not attached to the same controller. On Apple Silicon Macs it doesn’t matter since each port has its own dedicated controller and gets the full 40 Gbps b/w.