empty trash only on external disk?

There is one functional reason. The more contents a single folder has, the more likely you'll get data corruption. A more smart system keeps your data backed up on a regular basis, instead of using your external hard drive for additional storage*:

http://www.macmaps.com/backup.html

And then once you are certain you don't want the data on your drive to be taking up space, you burn it to CD or DVD at least twice, or keep it on a second partition of the hard drive as well as a CD, until you are ready to get rid of it. Keeping it in just one place is risky. This is especially true since you can always accidently empty the trash on the only copy of important data you want to keep. Once you have agreed to maintain a good backup system, I can give you instructions on how to empty your trash. Because there is always risk if you don't do it.

- * Links to my pages may give me compensation

it's possible but the system is certainly not designed for this which is why you are having problems. There is no GUI way to do this so it has to be done from terminal.

run the following command in terminal:

*rm -rf /Volumes/"drive-name"/.Trashes/`id -u`*

Put the name of the drive in question in the above and keep the quotation marks.
However, I would recommend you adopt a different system for long term storage of your "maybe" files.

yest, the command deletes the .Trash folder entirely.
what you say about having to remount the drive is strange. that .Trash folder should be recreated as soon as you try to trash something on that drive. that's how it has always worked for me.
I suppose you could amend the command to
\\
rm -rf /Volumes/"drive-name"/.Trashes/`id -u`/*

This will delete everything in the trash folder but not the folder itself. I didn't do it this way in my first post because this should not be necessary.

For your maybe files, just create a folder somewhere outside of the trash can folder. Call this folder "pending trash" or something along those lines, then move the contents of your trash to that folder. Put the contents of the second disk in the trash, then empty it.

Apple designed the trash folder for the purpose of deleting files. If you are uncertain about deleting a file, it has no business being in the trash; it should go in a pending folder.

While it may be possible to remove just data from the external disk, you really should not be using the trash as a maybe folder. Create a new folder on your desktop for that.

Be extremely careful with rm -r. best to do an rm -ri (that's the letter I in lowercase) to ensure you don't delete files accidently.

you are right but the man says he has several GBs of stuff in his Trash.
Confirming that he wants to delete every single file one will take forever.

you are right but the man says he has several GBs of stuff in his Trash.
Confirming that he wants to delete every single file one will take forever.

If its in fat32 mode, you could plug it into a PC and delete the trash folder that appears there.

You could create a PC partition on your MAC and do it. :P