How to get over "device or resource busy"?

The tool you want is lsof, which stands for list open files.

It has a lot of options, so check the man page, but if you want to see all open files under a directory:

lsof +D /path

That will recurse through the filesystem under /path, so beware doing it on large directory trees.

Once you know which processes have files open, you can exit those apps, or kill them with the kill(1) command.

sometimes it's the result of mounting issues, so I'd unmount the filesystem or directory you're trying to remove:

umount /path

I use fuser for this kind of thing. It will list which process is using a file or files within a mount.