I just got a new Mac Mini server with Mountain Lion. I used the Migration Assistant to try to move files from my iMac to the server. Two problems - first, the Migration Assistant is too much of a "blunt instrument" - you can't pick files and folders to move, just broad categories of content. This ***** and leads to importing far more than you want to.
Second problem, the HD is now full on the server, and I can't make the icon appear on my desktop, so I can delete surplus files that I didn't really want to copy to the new machine. Oh, and the purpose of the file transfer was to import my iTunes collection into the new computer, which Migration Assistant failed to do.
So I try to delete unwanted files, and can't get the HD icon of the boot drive to appear, except in a window that won't let me throw away any of the files, because they are "in use".