Disk Utility First Aid has no progress bar

How big is your hard-drive? It's entirely possible that it would be normal for zeroing out the sectors to take that long. I'm only extrapolating here, but I've had secure-deletions of the trash take an hour or more for a few gigabytes, so if the hard-drive is large, I could see it taking quite some time.

The other thought is that there is some problem with the drive that prevents the format from completing. You might want to check the drive for errors and/or bad sectors. It is possible that you would have eventually encountered the error on the hard drive yourself (potentially causing instability, data loss, etc.).

Finally, if you look at Activity Monitor, does it look like the disk utility is doing anything? Check your disk I/O statistics there as well and see if anything is still going on with the disk in question.

You can force unmount a drive by running the following Terminal command:

diskutil unmountDisk force /Volumes/VOLUMENAME

^{Replace VOLUMENAME with the name of a volume on the disk you are trying to unmount.}

Following this, attempt to Erase/Partition the drive again in Disk Utility. Using the above command can interrupt file read/writes, which can cause file corruption. However, since you are erasing the drive anyway, this doesn't really matter in your situation.