This is a fairly basic how-to, if you’re already familiar with Linux and/or OS X.
There are many different ways to empty the “Trash” in OS X; there are keyboard shortcuts, menu options, right-clicking magic, etcetera.
All of these methods, however, will permanently delete trashed files on all the mounted volumes (disks). E.g., the standard “Empty Trash” will kill everything in Trash from your standard Mac’s hard drive (Macintosh HD), as well as things you’ve trashed off your iPod, and everything in Trash from that USB thumb drive… if you happen to have them connected to the machine.
What if you want to kill only the trashed files from the iPod to free up some disk space?
The only way I know how to do it is to launch the Terminal.app (inside Applications>Utilities) and manually delete the files directly from the command line. For example, let’s say your iPod is named “My iPod”. You’d navigate to:
… and tell it to delete the files therein.
I’ve purposefully excluded the actual commands that you’d issue in Terminal to do this… to save some poor n00b from accidentally nuking their shit and complaining to me that I fucked ’em over.
Anyway, every mounted volume has its own “.Trashes” directory. Do what you need to do, and free up the disk space on that other device, and save the trashed files on your main HDD that you just might need tomorrow.