I rip CDs and buy downloads on one PC, then copy the files to the MC computer either through the network or via a USB stick. Many times MC's auto-import will spot a new folder and import the contents before the network transfer has finished. It will eventually find all the files, but if you update the cover art before that happens the last file won't get the update. The work-around is a bit clumsy, but before updating any tags on new albums I always check that the last track has been imported, and if it has not I do something else for a while or run a manual import on that folder only.