After reading the case of the missing El Capitan hard disk space and following all the advice in that column, Jim Williams still had an inexplicable 300GB that had no reason to exist. He ran a disk analysis program that showed that a hidden Unix directory named .MobileBackups.trash What is it and can he get rid of it?
This is a side effect of Time Machine, of all things. When one of your Time Machine targets is a drive that isn’t currently connected to your Mac, the backup system will continue to generate system snapshots up until all but 20 percent of drive storage is filled. After that point, Time Machine starts to delete snapshots and is more aggressive if you have very little storage available (less than 10 percent of drive capacity or less than 5GB).