Same thing happened to me when I copied everything to a new sdcard. Takes a while to find it. Strangely enough, I used the stock music app to play a couple tracks & that seemed to "teach" Google music where to look. Had the same thing on the stock gallery after I used SdMaid to clean some app caches. I accidentally cleaned the thumbs in the moto gallery & couldn't get em back til I pulled & installed the aosp gallery from my eclipse zip. Once the aosp gallery loaded the thumbs, they showed back up in the stock gallery too. Weird...
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