Do not split a RAID array across drives in separate USB enclosures.

Doing RAID on USB drives is alright, as long as they’re all in the same enclosure and use a single USB interface. If you split an array between drives with separate USB interfaces, you will face corruption and rebuild issues when one of the controllers has a hiccup or comes up slower/faster than the other, which WILL happen. If you need to run a RAID array on USB-connected drives, use a 2-bay USB-connected DAS. I’ve used the QNAP TR-002 in the past, it works fine, just set it to individual mode.