Here is a quick UNIX shell script which will empty the Trash on a single volume (disk) without disturbing the contents of the Trash on other volumes. I use it to empty the Trash on a Zip disk without disturbing my "main" (hard disk) Trash. As an example, if you name the shell script dumptrash, you could then empty the trash on a Zip disk called "Zip disk 1" by typing dumptrash "Zip disk 1". Remember to put quotes around the volume name if it has spaces, as in the example above. Your administrator password will be required for this operation.
#! /bin/sh
# Selectively dump the trash on named volume $1.
cd "/Volumes/$1" # produce sensible error msg if input volume name not found
sudo rm -frv .Trashes/*
[robg adds: Remember to make the script executable with chmod +x script_name and store it somewhere on your path. I have not tested this script...]
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