I dont think just by connecting a USB to the drive where the backup is stored will improve the Space issue. yet again i'm not sure about that...but first check...
i"m using the SQL server 2005 maintenance plans in place to take the backups and never actually encountered this issue. we have SQL server 2005 server SP2 service pack on...
Check if the servers i over loaded. It sometimes usually takes that long....and sometimes it completes with in less time. So we can't actually speculate the exact time it takes...