What is USM?

USM, short for Universal Storage Module, is the standard specifying how to put a SATA hard drive into a chassis so that the hard drive can be used with various USM-enabled recievers, like NASes, TVs, and so on.

The use of USM, in particular, allows external drives to be swapped in and out of the NAS for backup purposes.

How is it relevant to NAS recovery?

The USM is not really relevant to NAS recovery. It is just a feature often found on Seagate NASes.

If you have a slot in your NAS labeled "USM", leave it as, no matter if there is a drive in it or not.