Migrate from 5N to 5C? Or Erase a drive?

I just bought a 5C and have been using a 5N for a couple years. I don’t like how long everything takes and don’t use the network aspect of it, so I wanted to simplify to a direct-attached device in the hopes that accessing files and backing up will be faster. I expected to be able to just move the drives from one to the other, but received an error that the firmware in the disk pack and the 5C are not compatible. Both are fully updated. Looking online I can only find info on migrating from direct-attached to direct-attached and networked to networked so I’m guessing this is not a resolvable issue.

My next plan is to pull two of the drives from the old, take the several-day process of transferring the data over the network one last time, and then move the remaining drives. However, I don’t see any obvious way to erase the disks I moved to the 5C so that I can start to use them again. How do I reset them?