iPod touch Syncing cancels in the middle

When I hook up my iTouch, it begins to sync like normal. I have bought apps on the iTouch, so when it hooks up, it says transfer the new purchases to computer or they will be deleted. When I sync, it says it copies all the apps, but when it is actually syncing...a message pops up saying the required disk cannot be found.

What do I need to do, because I have apps that are either purchased, or are not available anymore...

Thanks!

it also says the disk cannot be read from or written to.