Dwight Schoffer wants to know the downside of backing his data to a flash drive, then deleting it from his hard drive.
Making one backup, then deleting the file from your hard drive, is no better than not backing up at all. You're still violating Spector's First Law of Computing: Never have only one copy of anything.
And if your only copy is on a flash drive, that's especially dangerous. Those things can easily get lost or stolen.
If you're going to keep files on a flash drive and not also put them on your hard drive, you need to back up the flash drive. Copy the files to another flash drive every day or so, or burn them to a DVD.