To me this sounds like a defective external harddisk.
The slowness of the system when you plug the disk in, is caused by it trying to read from the disk, as it recognizes it as a hard disk, and wants to read its contents, -even if you do not try to directly access it.
(You could check the eventlogs, and you will probably find a lot of errors related to the disk.)
Disks in a state like this probably have bad sectors on them, where the disk finds an error, and tries to re-read the sector, and re-read and re-read... this will take a lot of time. Should you be patient and wait and wait and wait, eventually you will get an error popping up, or in the end it may read the disk correctly.
But chances of that last thing happening are very small.
To try and save/retrieve data from it, I would try and hook it up to a different system, running a different OS (like Windows 7, or better yet Ubuntu or some other Linux variant. There's a small (very small!) chance you will be able to access files on it that way.)
Them external disks are nice, but also kinda prone to fail.
Especially during startup and shutdown (power on/off) and plug/unplug.
(Just yesterday at work I had a 1TB hard disk go fail, just because a disconnect / connect , it somehow wiped the entire contents of the disk... it was empty, unformatted... oh well ... I could re-partiton and re-format it in the end)
Your problem however sounds like a physical defect... to me it looks like disk and data will be lost...
Wait for others to chime in....