Same problem but different phone
I'm having the same problem with a Xperia X10 Mini. I copy mp3s to phone using USB cable and the songs can be played from phone on PC. When I disconnect phone (having 'safely removed hardware', unmount SD card etc) and open music player, there is no sign of some of the albums and songs.
When I plug the phone back into the computer, Windows explorer shows that although all the folders (ie album title) are there, most of them are empty.
I've tried loading the card by using my camera as a card reader but the same thing happens - songs can be played from the card , but when removed and installed in phone, the files are deleted.
I have only noticed this problem today, and it coincides with installing a new (8GB SD HC) card.
Any help much appreciated before I ditch Android, sell the phone and go back to my old W705.