Remember that ‘syncing’ means making your player the same as your computer’s hard drive library. So if you have, for example ripped a buch of CD’s to your hard drive, then ‘sync’d’ them to your Sansa with Windows Media Player, then gone back and deleted them from your hard drive (because you no longer need them there as they are now on your player); then the next time you plug in your Sansa to your computer, Windows Media Player is going to look and see if anything has changed. Why, yes it has . . . and you guessed it, it’s going to remove everything from your player that doesn’t match what’s on your computer any more.
Yes, you can turn off ‘automatic’ and go for ‘manual’ sync, but this could still happen. This actually happened to me, and I quickly switched to MSC mode and ‘drag & drop’ and have never looked back.
“Fool me once, shame on you. Fool me twice, shame on ME!”