If you can see the disk from the Disk Management console, try a format again and see what happens.

Sounds weird... can you put back the files on your XP system, with the empty external HDD go to the Vista machine and format it from there. Go and put the files from your XP to the External HDD, and see if Vista can see them

"When I go to the Computer Management option the external disk is shown as "Unitialized". What exactly does this mean,,,,,? "

 

This means Vista thinks it is a new raw unformatted hard drive, whatever you do do not initialize it, data loss will occur. See #5 here  http://www.seagate.com/www/en-us/support/troubleshooting_assistance/internal_drive_interfaces/windows-vista

 

I have seen this on other posts, but have no solutions, sorry. It is probably a conflict with the usb chip in the external enclosure and Vista.

 

 If there is No data (or data you don't care about) on the drive, you can initialize it in Disk management, and then partition/format.

 

Others may post with more info.

You are welcome, Thanks for posting the Fix, this helps others searching for answers.