Pro6 back to Readynas after using alternative OS

Are you still able to get it to boot into FreeNAS? USB Boot Recovery isn't the only possible way to attempt to fix this if you can still boot into FreeNAS.

To put OS6 on you'd need to use the RAIDiator-x86 4.2 USB Boot Recovery and rename the OS6 firmware to e.g. RAIDiator-x86-something and put that on the key so that it will use the image.

If you use usbrecovery.exe to create the syslinux bootable USB key, be sure to run the tool as an Administrator. Also make sure the USB key is using the MBR Partitioning scheme with a partition table with a single partition. USB Boot Recovery looks for the firmware on that partition. Not all USB keys are compatible so you will likely need to try multiple different USB keys.

As you have wiped the internal flash you'll have wiped the EEPROM, so the system won't know which ReadyNAS model it is. So that's another thing that would need fixing.

Unless booting off USB the NAS always boots off the internal flash in the early stages of the boot process and for some boot options it'll then chroot onto the disks.

Were you booting FreeNAS off the internal flash or something else? Messing with the BIOS settings shouldn't be necessary if you haven't changed any BIOS settings.