If you don't do it properly, you won't achieve vendor mode and will be unable to read/write to the drive. So unless you physically damage the drive in some way, not much to worry about here, other than hooking the power up inverted.
You should send vendor outro, to take the drive out of vendor mode. After you have achieved write verification. All though power cycling the drive accomplishes this too. Still send it. After it's sent, if everything was done correctly, where the drive properties display Vendor Mode, will change to the inquiry string of the firmware you just flashed. Then you'll know it should work as soon as you put the piece of poo back together.
EDIT:
**Don't pay any attention to this posting if you're playing with Hitachi's, they have their own "rules" that must be followed. Or you will end up with the only type of drive that get's bricked from a bad flash.***
This post has been edited by insanity4all: Jun 27 2010, 01:14 AM