They will not be unrecoverable if moved, but they will be deleted as far as the OS is concerned. A move command is effectively just a copy command combined with a delete command all done by the system. It has the same effect as if you copied the stuff and them manually when and deleted the stuff yourself by way of the Recycle Bin (in Windows) or Trash (in macOS).
Now, when you do a typical deletion of a file in most OSs (this includes any version of Windows and macOS), it is not actually completely destroying the file on the hard drive or SSD. The file actually will remain, but the file table that effectively keeps track of the file for the OS has the entry for that file removed. So, while the file is still on the drive, the OS thinks the file is gone because it has been removed from the file table. This then allows the OS to be ready to write new files to the space that the old file is sitting in. Until that old file is actually overwritten, however, it can still be recovered. Think of the drive like a book. The file table is like the index or table of contents. The file is like a paragraph or sentence or word or chapter in the book. So, in the book case, the deletion of the file is like going a removing that chapter from the table of contents or word from the index, but the word/chapter/etc is still actually in the book. You cannot easily find it, but it is still there.
If you want the content unrecoverable, then you need to use some sort of secure deletion function that will overwrite the space that the old file(s) occupied with other content, typically random ones and zeros (it is a binary thing) for many such secure delete functions. What option you have for this would then depend on what OS you are using and whether it has a built-in function or you might need a third party tool. And I am not sure if any of such functions or programs would work with a move command other than after the fact (i.e. goes back and securely wipes all empty space on the drive or something like that).
As a side note, I would tend to recommend doing a copy and then later deleting (whether secure or not) when "moving" files from one drive to another. This allows for the possible problems that might happen (although unlikely typically) during the move. If you use a move command and something goes wrong, the files are effectively gone potentially from both drives (you could potentially recover the files from the original drive as implied above if not securely "moved", but that is more hassle). If you copy and then manually delete later, you can then first be sure the files end up on the new drive BEFORE you delete them from the old drive.
Edit: You can make the files unrecoverable if using a strong enough secure delete function/program. There are effectively military grade wipe programs and functions that even mere mortal citizens can use. And if the military is comfortable with them, then you can be sure that the data is unrecoverable (at least by current means and likely by most if not all future means...but I will not say never as I don't know what the future might hold...my time machine is currently on the fritz ).
Edited by hamluis, 25 May 2017 - 03:09 PM.
Removed unnecessary quotebox - Hamluis.