Hi Moe,
This issue has me baffled!.... No one seems to know the answer!...
I get the same thing and some of my EXS Intrument Samples in my Logic Library:
Then, I look for the file in Finder and the sample exists:
Im no computer expert and I don't know how to link the 2 together :-(
If often thought I may have something to do with the 9.1.7 update or:
I've tried "MacKeeper" in the past to clean up my drive and my Logic Studio App's freaked out. I remember i had to re-install the updates for both Mainstage and Logic after a system clean. I read somewhere that the EXS sampler relies on 'file paths' to find it's samples, i've often wondered whether Mackeeper wiped them out and I somehow have to re-connect the samples with the sampler if you know what I mean.
Anyways, I could be way off the mark but it's a common problem and your not alone. I havn't tried a re-install yet, Im still hoping there's a simple solution.
Good Luck.