Problems transferring purchases from iPhone to iTunes

[Clo8]

I'm trying to transfer my purchases from my iPhone to PC before updating the software as this is what it recommends I do. However when I click 'transfer purchases' it tries to sync and then comes up with an error message saying 'Attempting to copy to disk 'Elements' failed. An unknown error occurred (-39).' Elements is the name given to my external harddrive where my music library is - iTunes recognises this and will sync with it, it just seems to have a problem with transferring my purchases made on the iPhone there. Please help!