Well, I've gotten through the last week without losing any more tracks. I switched off iTunes access to the NAS, but I don't know if that's the culprit, or not, but I've read complaints about iTunes playing big brother and after people have ripped their CDs. The accusation is basically that iTunes looks at the music you've ripped and goes, "Hey, that didn't come from us! It must be a pirated copy!" and promptly starts to delete it from the hard drive. I don't know if there's any merit to the allegation, but I don't need iTunes accessing my music anyway. I've also backed up my music on a separate USB drive, which I don't leave connected to the PC, just in case whatever has been deleting music files from the NAS decides to have a go at the USB drive, as well.