Please give us more info to work with?
Drive manufacture date/complete model #?
What is the size of the firmware backup file you made?
What program was used to create the backup?
What is the original firmware version on your drive?
What version/Where is the Plextor firmware from?
Are you willing to try this with Flash Utility?
Have you completely powered off the drive?
Sometimes a sata controller/sata mode issue can make this more difficult. You may wish to move the drive to another sata port to see if this helps.