See this post about the 4 day hold on trash: http://moodle.org/mod/forum/discuss.php?d=191275
We are having the same problem as described here and above. We'd been tweaking the automated backup code to fix the file naming back to the way it was in 1.9 (keeping the shortname in the file name). As a consequence of the hacking/tweaking, we've enabled and disabled automated backups several times for testing. Collectively our semester backups are about 25GB, so during this testing phase the trashdir folder has expanded significanly from the discarded automated backup files (see forum link above - we keep only 1 copy).
I'm not ready to tweak the db table just yet (as suggested in the post above) because I hate hacking the core or the tables. On a side note, there's also 6gb of stuff in the moodledata/temp/backup folder. I suspect this is how moodle processes "unifinished" backups, so more research is definitely in order.
While it was relatively easy to calculate file space needs in Moodle 1.9 based on your knowledge of site usage, I'm realizing that in Moodle 2+ you really need to plan to have a heck of a lot more available storage for the moodledata folder to accomodate the new file management system. I just wish where was a way to calculate just how much space that should be!