This could be a big steal for Google. iTunes Match is annoying in that it frequently refuses to match or upload one song on an album, meaning that effectively the whole album is broken. I've lost count of how many times I've found myself not listening to the song I expected because apple can't fix their service. I would completely switch to google if it works.
The solution is to, frankly, badword about with USB disk mounting and unmounting. Most reliable is to turn off debug mode, turn on USB disk mode, eject USB disk, then go back and turn on debugging mode. If you get the warning OK/Cancel requester when turning on the debugging mode that seems to be the magic talisman to indicate that you have effected the appropriate mystic sigils to get to first base.