Wow.... that is weird... :serious:
let's start with what you're playing with... (I'll just assume you have a later model SV without the RF package). Your symptoms are odd to say the least. The Nissan HU can be a bit fussy with USB drives (both make & size). Do you have any others you can test with? What brand & size is the drive thats causing the trouble? It's also possible the HU is taking issue with the file structure. PM me a reminder to post/send you a program that will order/organize the drive's file structure (I don't have it here at work with me).
It's also possible the HU just needs a cold boot (or a boot to the head... you'd be surprised how often that fixes a transmitter at work). You can either unplug the stereo from the vehicle harness (difficult) or disconnect the vehicle battery positive cable for... 15 min to an hour (easy). TBH I'd be surprised if the cold boot did anything but it's usually the last or first thing I try, depending on time and frustration level.