Try plugging the drives in and then going to the Finder Menu and emptying the trash. The drives are probably full of deleted files that are still there, and until the trash is empty, they still take up disk space even though you can’t see them.
It might seem unintuitive for a computer to work that way, but that’s the way your office trash can works: even though you may throw papers in the trash, they are still there until you empty the trash. That can be handy on your Mac if you regret throwing away a file, because it’s still in the trash and can be dragged back to the desktop.
And there are two options for the OS X trash you might be interested in: one to turn off the warning that asks if you really want to empty the trash, and an option to securely empty the trash.
In the Finder, go to the Finder menu and select Preferences. In the Advanced Tab, you ‘ll see those options, among others. You can leave the empty trash warning on as a reminder to yourself, or turn it off. And you can check “Empty Trash Securely,” which means that files that are in the trash wen you empty it are overwritten with random data to prevent them from being recovered with disk utilities that can “undelete” files.