Force Finder and Trash to stop using external drive so Disk Utility can erase I want to reformat an external disk, but in Disk Utility "Partition" is grayed out and when I click Erase it says it cannot unmount the drive.

I think Finder is not allowing the disk to be unmounted because I put a large directory from the external disk in Trash, told Finder to empty the trash, then cancelled. Now there is no option to "put back".

I'd rather not wait for the whole directory to be deleted from the external disk. Is there any way to force Finder to forget about the trash from the disk and stop using it, and allow Disk Utility to unmount it?
I shut down, unplugged the drive, started up, logged in, plugged in the drive and quickly opened Disk Utility and clicked erase, and it just worked.

I'm not sure if it matters that I did that fairly soon after logging in.

I'm not sure if it matters that I didn't open a Finder window or the Trash.