Since installing Mountain Lion my 2009 Macbook no longer recognises an external disk
Before installing Mountain Lion recently I backed up my 2009 Macbook using Time machine and my usual external WD 1To disk. I back up monthly as the Mac is for leisure purposes only and I connect my external disk as and when necessary. I tried to do this today and Time machine stated that the registered disk was not available even though it was visible on the configuration pane. Thinking it would be easiest to start again I deleted the disk but now I cannot specify it as the back up disk, only time capsule, leading on to airport being available. I cannot see the disk on a Finder window either.
Can anyone help me to 'find' my disk and re-configure Time machine?
MacBook, OS X Mountain Lion (10.8.2)