Been using like 7 external usb drives with 40-50tb total for a few years with no issues. Not raid, just backing up drive to drive. No controller or drive issues. Mix of seagate and wd 8/12/16gb.
I hate blanket recommendations like this by docs. To me, it just sounds like some guy had a problem a few times and now it's canon. It's like saying "avoid Seagate because their 3tb drives sucked." Well they did, but now they seem to be fine.
Yes I should have specified that this advice is specific to RAID configurations in NAS applications.
If you're occasionally copying data to an external USB drive, that's totally fine. That's what they were designed for.
The issue is that they were not designed for continuous use, or much more demanding applications like rebuilding/resilvering a drive. It's during these applications that issues occur, which is a double whammy, because it can cause permanent data loss if your USB drive fails during a recovery operation. I did a little more research after posting my last comment and came across this helpful post on the TrueNAS forums going into more depth: https://forums.truenas.com/t/why-you-should-avoid-usb-attach...
YMMV. I have a 4-drive 20TB mdraid10 across two different $50 USB3.0 2-drive enclosures, I've read petabytes off this array with years of uptime and absolutely zero problems. And it runs on one of those $300 off brand NUCs. The 2.5G NIC is the bottleneck on reads.
Is that with ZFS or something else?
Mainly I wouldn't do it because of there's space and SATA ports it seems stupid. Hotter. Worse HW.
Can't really see much good reason to do it tbh except it's in a small hot case which is relatively easy to move around. Maybe if you do occasionally backups and you don't care about scrubbing and redundancy? Otherwise why not shuck them and throw them in a case?
What may work anecdotally can't necessarily be used for official recommendations for a large range of users across an unknown range of hardware configurations. If it works for you, that's fine. That isn't sufficient to make a general statement that everybody will be fine using external USB drives, particularly for RAID, especially when people will then make you responsible if something goes wrong for not making sufficiently safe recommendations. You understand that, right?