For these devices (and most Terastation models) you don't need a recovery image and don't need to mess with TFTP because it already has the recovery image built into flash memory on the device.
There's a slight chance the problem you're experiencing is just with the boot partition of the first drive, before trying something more complicated you can try swapping the positions of the drives to see if you can find one that will successfully boot.
If you don't care about the data on the drives the easiest way to proceed is to just wipe the drives (or at least remove the partitions) then re-insert them into the device. Then when you boot it up it will go straight into EM mode. From there you can use tsupdater to repartition and reformat the drives and then re-install the firmware.
See:
https://buffalonas.miraheze.org/wiki/Enable_Debug_modeIf you don't want to wipe the drives you could remove the drives and start the device (I believe it will still make it to EM mode that way). You could then re-insert the drives and attempt to re-install the firmware without reformating/repartitioning (which should preserve your data provided there's nothing wrong with the data partitions).