Let me guess.
There was some software on the disk/you downloaded like WD Security or Drive Utility and you used password protection.
But right after you removed the software from a computer –> the password is incorrect. Yes?)
If that so, you can try luck by installing the software back (install->run–>close->connect disk->run again) Also, under that software check menu bar for preferences, options, etc. for something similar to password reset
I didn't try the abovementioned. Lost all my data(