ready nas rnd-2d not working, need to access files on discs

Any current ReadyNAS should be able to access your volume, assuming that the volume isn't the problem you have now.  So it is a good idea to do some troubleshooting, even if you decide it's time to upgrade the NAS, before you declare it's the hardware that's the problem..

If you move the drives from the old to the new, with power off, and then power on, it should mount your old data volume and all will be well.  It is best that the new NAS is on the same OS version as the old, though it really shouldn't cause any serious problems unless your old NAS firmware was way out of date.  If you have apps installed and you move to an Intel based NAS, they will not longer function.  We normally recommend you remove them on the old NAS before the migration, but that's not possible if the old NAS is broken.  So, you'll have to try to remove them once the volume is in the new NAS and then replace them with an x86 version if you still need them.