External USB Backup-How to access it

Replacing just the main board is risky where there is such significant damage, as there could be secondary damage of the small daughterboard, SATA backplane, and/or power supply.  You should also check the drives using the manufacturer's tools.  When you do look for a replacement, be aware that the RNDP6000-100 and RNDP6000-200 have very different processors but are otherwise electrically identical.  The earliest -100 also shipped with the older round-button caddies, but you can use your old ones, anyway.  The CPU in the -100 can also be upgraded.

You are not limited to another similar model, either.  Since you already upgraded to OS6, any ReadyNAS with at least 6 bays will accept your volume (assuming it's undamaged)  and your old-style caddies will fit even the newest models (since many are sold without caddies).