HTC :: HERO Keeps Deleting Music

I decided to revamp my MP3 library, so I deleted all the MP3's off my phone and then copied a whole new set onto the card. All seemed to work fine, no errors on transfer, all the MP3's showed up in the appropriate folder on the card, and the few I tested (by playing them on my computer) worked just fine. I then disconnect from the computer, and let the phone "remount" the memory card. The phone gives a message saying "Preparing SD card". Unfortunately, it seems that when it says "preparing", what it really means is "DELETING random MP3's". For some reason, it will only let me keep SOME of the MP3's I copied onto the phone less than half of the number I had on it before I deleted them. I can actually WATCH it delete the files. I go to the music player, and watch as the song count spirals downward, my MP3's fading away into the ether. I've tried copying via HTC Sync, and I've tried by copying straight onto the card via a card-reader slot. The results are the same:

I copy the files onto the card
I verify that they are present and working on the card
I insert the card in the phone (either physically, or by disconnecting Sync)
The phone mounts and "prepares" the card
The MP3s I've just added have simply vanished
I re-Sync, or check the card via card-reader
The MP3's are gone. Not in the file, nowhere on the card.