"RNDU4000|ReadyNAS Ultra 4" Stuck on Booting... during factory reset stage

Is this a new setup (perhaps a used purchase), or was the NAS working before?  If it was working before, was there a reason you wanted to do the reset?

First I suggest checking the procedure in the hardware manual, and make sure you did a factory default - that would be pages 21-22 here: http://www.downloads.netgear.com/files/GDC/RNDU2000/ReadyNAS_Ultra_UltraPlus_NVX_Pro_HW_en_8July11.pdf

Once you've confirmed the procedure, a good next step is to test the disk drive.  You do that in a Windows PC (connecting the disk either with SATA or a USB adapter/dock).  Use Seatools for Seagate disks, Lifeguard for Western Digital.  I suggest running the destructive "write zeros" test after you run the usual long diagnostic.  That can pick up problems the non-destructive tests miss.  Also, the NAS should then automatically do a factory install when you put it back in the NAS - you won't need to use the boot menu, as the disk will be blank.

While the disk is removed, you can also power up the NAS, and confirm that you see a "no disks" status on the LCD.  Also install RAIDar on a PC, and confirm that it can discover the NAS (also reporting a diskless status).  https://kb.netgear.com/20684/ReadyNAS-Downloads#raidar