What happens when you force quit a `mv` directory operation between two disks?

When moving a directory, all of it's contents are copied before the original gets deleted. In other words, yes, it is safe to interrupt a mv command when moving a single source, such as a directory - even though it might have files and subdirectories, it's still a single source.

Note that this is not the case when moving multiple sources with one command, such as:

mv a.txt b.txt /somewhere/else/
mv *.txt /somewhere/else/

In these examples, files will be moved one by one, so if you interrupt the command, you will not find the files that were already moved in their original place.