you want to back up your hard drive to a flash drive?
The largest flash drive i've seen had 32gb of memory..and it depends on how much space you use on the hdd.
you're really the only one who can answer your question. how much would you want to save in the event that your hard drive crashed? if you have say 100GB worth of files that you want to keep, it would be much more practical to buy an external hard-drive.
Dont back up your computer on a flashdrive, no offense but that's stupid. You could buy some more hard drive for your computer but flashdrives can get damaged easily and get lost.