[chris_j]

What is it that makes USB perfectly fine for spinning rust but not quite ideal for SSDs?

I've been booting a couple of Pi 4s from USB SSDs for a while, in order to avoid relying on SD cards, but I'm not really familiar with what all of the implications of this are.

[johnklos]

Oh - I'm just talking about speed. Of course there's no problem using SSDs aside from that. I'm running several older Pis off of SATA SSDs connected via USB 2, for better speed and reliability than SD cards.

For spinning rust, two fast (220 MB/sec) drives don't max out the single USB 3 channel on a Raspberry Pi 4. I have one colocated with two software mirrored 8 TB drives, with UASP, and I can get the full 220 MB/sec per drive at the same time with that setup.