Trash Emptying Halting

I am deleting back-up files from an external hardrive. There must be huge files. The problem is that I keep getting messages "unable to delete the file, file is in use" and I have a choice to 1:Skip, 2: Stop, 3: Continue. I have physically monitor the computer and click one of these choices every minute or so or the process halts...I have about 20 hours invested so far.