Slowly but surely my entire iTunes library turns unusable. HD-failure, malware, or quirky DRM?

And it’s driving me nuts. My library resides on an external WD MyBook 500GB FireWire 400 drive. Daisy-chained to this drive is a WD MyBook 750GB for TimeMachine.

On, top, there is a TimeCapsule in my network that I use for my MacBook Pro. So, I was quite often copying my library back and forth before I found the right setup, and additionally, back it up to Mozy.com off-site back-up.

Now, since about a week or so, it seems that rather randomly some files simply become unusable. Trying to play it from iTunes simply results in…nothing.

It doesn’t start playing, but it also doesn’t give any kind of error message. It just sits there. This goes for iPhone and AppleTV as well.

The only thing that noticeably changes, is that the file icon in finder changes from the CD cover to a random *.mp4 icon (file with a blue note), but with a little padlock in the top left corner.

The files are there, get info gives all the info, they simply won’t play.

I’m really worried as I have a huge “purchased only” library. I don’t really want to experiment as the content is worth several thousand bucks and I’m worried I break it even more.

Any ideas???

Thx in advance