nMP + Pegasus2 + temporary hdd?

So I have a nMP and Pegasus2 in raid5. I received files for work on an internal HDD but not sure how to approach it. I think if I remove a HDD from the Pegasus and replace it with the loaner it will try to rebuild?

I just want to be able to copy the files from the loaner to one of the drives in the Pegasus.

Help me solve this problem, please.