Windows 7 & XP won't allocate drive letter

I'm having trouble with both Win 7 and XP recognizing my external WD 500GB HD.

I recently had to move the HD to a new enclosure b/c the control board was damaged while I was travelling. Neither my old computer (XP) which I had used with the HD and my new one (Win 7) which has never been connected to this HD will initialize the drive. Both see the drive and it is listed in Device Manager but a drive letter is not assigned despite using different USB ports, refresh, etc.

As per the new enclosure (Sabrent) directions and numerous threads, if this occurs, then I must format or initialize the drive in order to get a drive letter. However, the drive has about 40 GB of family photos, so erasing them is clearly not an option.

So how can I get either of the systems to recognize the drive without losing my data? Also, I'm completely miffed as to why the XP machine won't now allow the drive to be plug and play anymore.

Suggestions?

Go into Disk Manager and try and assign a drive letter in both XP and Win 7.

You can get to Disk Manager by right-clicking on the "My Computer" icon in Xp, and the "Computer" icon in Windows 7 and then selecting "manage computer" or "Manage."

Thanks Derek47 - but the HD is not listed in Disk Mgmt. I uninstalled and reinstalled it, and it still is not listed in Disk Mgmt and it prompts me to Initialize Disk:

"You must initialize a disk before Logical Disk Manager can access it"

I originally thought it was listed b/c it is the same size as my internal. Also, my other external, a Maxtor, was quickly visible.