Maybe I am overthinking this but I am curious as to the best way to move a large amount of data to another drive. I am adding a drive to a file server and need to move about 500gb worth of data. I searched around a bit and didn't find anything relevant so perhaps it is as simple as it seems.

If I were moving files on the same filesystem, obviously I would just use mv. I don't know how long it will take to move this much data between drives but I assume it would be a while and the system is at the mercy of the power grid, and I don't know if using mv would be safe in the case of power interruption. Does it copy the entire group of files and then delete them, or does it delete them as it goes? I guess the question is mv the same as a manual cp then rm? I am not cloning the entire drive, just moving some of it. What is the standard way of doing this in a professional environment where data loss is unacceptable?

I would probably consider using rsync.

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To answer the underlying question, don't use mv. cp or rsync give you a "free" backup you can delete at your leisure.
You can never have too many backups when moving (important) stuff around ...

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Regardless of the OS in use, as stated above, always copy then verify then, when you are sure the data is copied in full, delete the original.

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I would NFS mount, use tar or rsync to COPY (not move). Run an md5hash on each file and compare to make sure all copies are pristine before you delete from the original server.

tar is preferred over cp because it is better at preserving symbolic links and other file attributes. I think cp did not preserve symbolic links. I don't recall how well rsync's preservation works. Probably fine, but I remember the tar arguments better

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byau and 273 brings up a good point about holding data until verified. Rsync or some version is a great way to consider.
If this is a live state copy then you may need to stop access to the drive and then copy it. There are only a few ways to copy live state data and keep it all concurrent.

We assume you have a local disk to disk transfer.

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Yes, local disk to disk transfer. rsync seems to be the best way to go. Thanks everyone.