So I recently put all of the songs I have on my desktop computer onto a flashdrive, and then plugged the flashdrive into my laptop to transfer them all onto my iTunes account that I have set up for both computers. After I spent hours figuring everything out, I plugged my iPod in and attempted to purchase the songs off of the laptop onto it. While they were syncing, an error message from iTunes popped up and said that some songs "Could not be found" and it gave me a list of about 200 songs that wouldn't purchase onto my iPod. I can't even play them off of iTunes. Why did it do this? Any ideas?
Please help! :(

iTunes is looking on your flash drive for the songs (all that's in iTunes is a list of them). Is the flash drive still plugged in, and is it still showing up as the same drive letter?