Making backups on USB drives seem to only work if you're not switching PS3s as a lot of saves carry data from the machine itself and will not load if the machine they're in do not match their encrypted data.
I've been tempted to back up my save so I can delete it off my PS3 and put the back up back, to make sure I get the cars I want for my Lv 21-23 coupons, instead of having to keep creating accounts to try receiving a car I don't already own, but I am still afraid of just losing all of my data in the end.