Steam – How to copy an installed steam game on the PC to an external hard drive

Steam supports backup-files. Here is a part from the Steam FAQ/How-To's:

Creating Backup Files

1. Right-click the game in the Library section
2. Select Backup game files...
3. Check the boxes for any other games to backup at this time
4. Click Next >
5. Browse to the folder where you wish to create the backup files (the files will be stored in c:\program files\valve\steam\Backups by default) and click Next >
6. Select the Backup file name and set the File size for the media you plan to use.
7. Click Next > to begin the backup process.
8. Once complete, choose Open backup folder to move or burn copies of the backup files

Restoring from Backup Files

1. Install Steam and log in to the correct Steam account (see Installing Steam for further instructions)
2. If the backup files were copied to a CD or DVD, the process should run automatically when the disc is inserted. If not, run steambackup.exe from the disc
3. If steambackup.exe is missing, please download this copy of steambackup.exe and place it in the correct backup folder.
4. Continue through the Steam windows to install the necessary games.

Alternative to the Backup Feature for Third-party Games

1. Go to your \steamapps\ folder (by default, this folder is located at C:\Program Files\Steam\steamapps or on 64-bit systems, C:\Program Files (x86)\Steam\steamapps)
2. Locate any .ncf files for the game.
3. Locate this game's folder in the \common\ folder.
4. Copy all of the .ncf files and the game folder to a disc or available hard drive for reinstallation
5. For future installations, copy these files and folders to your new Steam installation.
6. Make sure to place it back in the proper folder (\steamapps\ for .ncf files and \common\ for game folders). In addition you may also rename/delete your ClientRegistry.blob to force Steam to rescan your gamedirectory.
7. Re-start Steam and you may see a small download to confirm the newly identified install.

For additional information read the complete article I linked below :)

Source: Steam FAQ/How-To's and Steam Glossary ClientRegistry.blob

A co-worker of mine had a similar problem when he tried to configure a Toshiba external for his Xbox. After configuring, his whole HDD was moved to the external, but he couldn't access anything on there. We ended up downloading his Live profile to the HDD and after that he could access everything on the external. Was this external drive for Xbox, or was it just a regular USB external? Do you still have profiles, themes, gamer pics, ect.? Were the files deleted immediatly after configuring the external? Did you check the external drive to see if anything was moved to there? When it stays on the loading screen, do you end up shutting it off, or does it shut off on its own? Get back to me and I'll try to offer some solutions