Instructions
Format your flash drive using a computer and attempt your backup again. Put your flash drive into your computer and right-click on its icon in "My Computer." Click "Format." This will erase all data on the drive, allowing the PlayStation 3 to use the full storage capacity of the drive for the purpose of backing up your data.
Log out of the PlayStation Network. Select the "Sign Out" option from your "PlayStation Network" menu on your console. If you're trying to back up PlayStation Network content, this content may be in use while you are logged into the service, which would slow down the backed up content considerably.
Close any games and stop any downloads that may be running on your PlayStation 3. Press the "PS" button on the controller and select "Quite Game" to close any games that are currently running. Highlight a download that is currently being transferred from the Internet and press "Triangle," followed by "Stop" or "Pause" to stop or pause the download. Attempt to backup your PS3 content again. If the backed up version is still running slowly, you may want to consider using a drive with greater storage capacity.