Thumbnail builds in R7 and R8

I have a new problem in both R7 and R8, at least I have not noticed it before.
In R7 each time I go to media gallery, the program has to re-build the thumbnails before the gallery will open. Why doesn’t it use the same thumbnails as previous?

In R8, each time I go to Media Gallery it presents an option to rebuild thumbnails. If I click on that option the program freezes and I have to use Cntrl Alt Delete to get out of the program. Is this a bug or is some other problem causing this?

Any help appreciated