iPhoto library grayed out, or disappears ...

Because my old iMac has such a tiny hard drive, I have to keep my iPhoto library on an external hard drive. This seems to create troubles like:
1) Sometimes when I start up iPhoto, the iPhoto library is missing.
2) Other times, the library is there, but it's grayed out.

I have a second external hard drive that I keep a backup on. My current situation is that when I try to open iPhoto using my original library, it's grayed out. When I open the library in the Finder, it is not. I tried copying the iPhoto library from my backup external drive to the original external drive, but it failed. Currently I'm using the backup version to look at my pictures, but I want to get the library back on my primary external drive before I add any pictures or make changes. I have about 10,000 pictures and I've already lost a year's worth of pictures and metadata on even more due to the library disappearing.

Questions:
1) What can I do to keep the library from either disappearing or becoming grayed out?
2) Why won't the library copy to the original external drive from the backup?
3) What can I do to resolve my current issue with the least risk of losing my backup version?

(Using iPhoto of iLife '08)

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