For Sync3, it needs to be formatted exFat. FAT32 won't work supposedly for Sync3.
I just upgraded my wife's Edge ST maps last night. I started the car, then once Sync was initialized and ready, I inserted the USB drive. Took about 20 seconds and then I saw the first message indicating it was updating. I think it went through 3 phases, Downloading, uncompressing, then installing. Took about 30 minutes.