Resolved External HD not registering in Disk manager

I have a external HD I use (well I did)

I have a win 7 Pro 64 bit Desktop. I have two internal HD’S installed Both are win 7 the second on was setup from a image backup of the 1st drive. ONLY one is active, The second one is unplugged at all time and is keep updated on win updates. I keep the second one for use if I get a virus. I can just switch drives
By opening up the case and unplug the bad one and plug in the second on and I and back up and running. Then reformat the virus one and restore it.
Here is my problem, I got a virus it looks like, I switched drives I am up and running.
The computer will not recognize my external Hard drive
It is formatted NTFS same as my C Drive. It shows up in Disk management but has no Drive letter assigned to it. I R click and get Error Reads that I need to refresh and restart which I do and nothing changes. I click I right click on the drive and nothing, It can’t recognize it to let me do any thing
IT shows my C drive formatted in NTFS, but not my External, Also I can plug in my other win 7 drive and I shows up and works fine ( that drive I having problems loading pages in Firefox I thinking a virus maybe I ran checks and no virus shows up I just want to wipe it and re image it back to be safe)

My two error read The operation failed to complete because Disk man console is not up to date refresh the view by refresh task if problem persist close DM and restart the computer
I get the same results after restart
When I right click it to assign Drive letter I get An unexpected error occurred check system event log for more info on error close DM and restart
Nothing happens after restart and nothing in sys event log about the error
Anyone have any ideas on what’s not right on this thing,
Thanks for reading

 

ok Update I got two External HD cases, Both are identical in every way both have the same type of HD WD caviar blue 500 gigs One External HD registers the the other one does not. I exchanged HD in cases the one that did not work, reads and works like it is suppose to with the Other HD and the one (HD that didn't show) did not work in the other case. which now ruled out the case as the problem. Leading me believe that it is the HD.
I then put the HD in the computer as a storage devise and it then showed up in disk manager with a drive letter of E I put it back in the HD external case and it is does not have a drive letter and cant do a thing with it in disk manager
I sure can't figure it out
This drive had windows Vista on it, I didnt use vista anymore so I used a formatting called ultimate boot cd 2007 which had a utility that over writes the HD that I used then put it in the external HD case

 

This is solved.
It was easy to correct. I went to device manager and in storage devices I had a unknown storage device listed I deleted it and rebooted and that fixed it had a drive letter after i rebooted. works as it should now, but what a headache it caused me
thanks