[CptnObvious]

I suppose all should be backed up to magnetic media (a HDD) as well

I use a USB 3.0 to 2.5" Hard Drive cable and HDD. Yes bootable and yes backup for multiple systems.

If you use 4 USB 3.0 to HDDs, you can have one Grandfather and 3 son backups.

Yes, I've been told that about USB flash drives need to be plugged in periodically to keep them alive.

[Paul R.]

I think it’s more than “need to be plugged in” — I believe the data has to be rewritten. Maybe someone has come up with a flash drive “refresh-files” utility...