What is using my USB drive?

For a while I’ve been trying to find out what is using my external USB hard disk on my mac when I want to eject it. Its been driving me batty and I usually resort to a few choice expletives before pulling out the cable anyway. Recently I read about the lsof terminal command - for example: lsof /Volumes/BACKUPHD This shows you what processes are currently busy stuffing around the specific volume....