I too have this problem. I spent a couple weeks uploading all my CDs
and all the music from 3 different computers into Google Play. I
believe that seemed to work ok, although I had A TON of doubles that I
had to delete. As well as a bunch of music that must have had incomplete TAGS, as the were not categorized.
Now that I took the time to get everything all straight, I want to sync
the music onto all 3 computers by downloading the music back to each one
so I have a complete collection each place. I have all my songs in
Google Play, but I cannot get them to download to any of the computers.
The best I seem to get is blank folders, or maybe a handful of tracks.
I let the Music Manager run for days with no luck. It seems to
reallllly slow my machine down too...this is something I wouldn;t want
to run constantly. But no matter how long I run Music Manager for, it seems to stay at "Preparing to download".