I forgot my password to my Free Agent GoFlex 500 GB hard drive. How can I retreive my data?

You could try this software it sounds pretty good although I have not used it so I am unable to direct you on how but it reads as though it has a password reset tool as well as windows password cleaner and a blank the password utility on any user that has a valid account on windows system and many other tools you might find helpful http://majorgeeks.com/Dr._Freeware_System_Utilities_d6416.html Dr.Freeware System Utilities which includes data recovery software, data backup, password reset tools for Windows helps you to maintain your system. Dr.Freeware is freeware, you can download and use it for free.
Data Recovery/Data Backup: Supports Logical scan and Raw scan, Supports NTFS, FAT, UFS 1, UFS 2, EXT2FS, EXT3FS, and ISO 9660 file sytem, Supports IDE/SATA/USB Hard Drives.

Data Recovery - Recover your lost data
Available in Live CD to run offline
Recover accidentally deleted files in your hard drive
Recover data from RAW Drive(partition table or file system is damaged)
Support NTFS, FAT, UFS 1, UFS 2, EXT2FS, EXT3FS, and ISO 9660
Support IDE/SATA/USB Hard Drives, memory cards, "USB Memory Drives"

Drive Dumper - Backup your hard drive
Available in Live CD to run offline
Available to run on Windows
Support Windows 2K / XP / 2003 / Vista / 7 (Windows Editition)
For data rescue, backup, migration, or computer forensics etc.
Transfer important important data from damaged Drive(partition table or file system is damaged, has lots of bad data blocks)
Support NTFS, FAT, UFS 1, UFS 2, EXT2FS, EXT3FS, and ISO 9660
Support IDE/SATA/USB Hard Drives, memory cards, "USB Memory Drives"

Windows Password Cleaner
Available in Live CD to run offline
This is a utility to blank the password of any user that has a valid (local) account on your Windows system.
You do not need to know the old password to blank the password.
Support Windows NT/2k/XP/Vista
Support IDE/SATA/USB Hard Drives

only trying to help

Hope this helps you