I have never seen anything like that happen, but I am no "expert."
It occurred to me that you might try running repair disk on your external drive. Candidly, I do not know whether that will help either, but it might be worth a try.
You might also look at these articles about Time Machine problems to see if any fit your circumstance:
https://discussions.apple.com/search.jspa?resultTypes=&dateRange=all&peopleEnabl ed=true&q=time+machine+backup+grayed+out&containerType=&container=&containerName =&username=&rankBy=relevance&numResults=15
Thanks for the suggestions; you are more expert at this than I am! I followed the link and someone who had exactly the same issue a few posts down was directed to https://discussions.apple.com/thread/2057525?threadID=2057525, but unfortunately all the links on that page point to MobileMe (with resulting closed sign).
I'm beginning to be VERY glad I made a manual backup on October 2nd - losing 6 weeks of work on this (prob about 15 hrs) is bad but is better than years worth of work. I don't know whether it is even worth bothering re-setting up Time Machine if this is how it "works," I doubt I'll ever trust it again.
Does anyone know of an archive / mirror of the Time Machine troubleshooting stuff which is no longer available on MobileMe? The file on the backup is over 700 GB so I know the 480 GB iPhoto archive is in there; just no way to access it.