Finally got thumbnails rebuilt on my very large 20K gallery

I have had broken thumbnails for a couple of years now and my browser always timed out about 500 into the thumbnail rebuild routine for a 20K photo gallery, so I had just given up. I finally tried what I found in the link below and was successful in rebuilding all 20K of them. It took me 10 passes along with following the procedure to first clear the cache using the special maintenance menu (NOT the normal admin menu), then I keep starting over again after each time-out of the rebuild process. A random error can pop up during any iteration of the rebuild process and when that happens, again first clear the cache from the special maintenance menu and then start again on the rebuild process. (Note that after an error, until the cache is cleared again, the rebuild process will likely stall again.)

The maintenance menu varies per your installation but can be reached by trying one of the following:
http://(yourgalleryURL.com)/lib/support/index.php?cache
or it might be
http://(yourgalleryURL.com)/(yourGallerySubDirectoryName)/lib/support/index.php?cache

So the short story is, always use the back button and start over on any procedure (don't simply refresh the browser). First clear the cache by going to the special URL for it (NOT the admin menu), then keep moving forward with the rebuild process until the browser times out again, then go back a screen and do it again. If you get an error, clear the cache again and then start the repeating rebuild process again. Each time the progress should rush quickly to the last failure point and then pick up again where it last stopped off. If it doesn't and it wast doing well at the beginning, try clearing the cache again.

Here is more info:
http://www.chipkillmar.net/2009/02/04/rebuilding-thumbnails-in-gallery-2/