but how would i be able to tell which one?

Divide and conquer. Take all the ones you added recently, and move them to a different place on your hard disk, a place that the Rio software doesn't scan.

Then, if the Rio software works, move half of them back. If it still works, move half of what's left back. Etc. It should only take a few tries to locate the exact file that it's choking on (if, in fact that's the problem... I'm not saying it is for sure.).

When you find the file (if it is a problem with one file), it's likely something unusual about the file itself. For instance, something corrupted in the file or something odd in its ID3 tags. Once you find the file, it would be nice to post here what you find out about it because that would be helpful to the software people so that they could correct the problem in future release of the Rio software.