You are correct, that the TSN is burned into the motherboard, but it's probably easier to move parts from (B) to (A):
I would start by moving the drive (temporarily) from the unsubscribed TiVo (B) to the non-working TiVO A. It's imperative that you also unplug the external drive before booting it back up. If it boots, you'll have confirmed that one of the drives in TiVo (A) has failed.
If not, then the next likely culprit is the power supply. I would then move that from (B) to (A) -- be careful as there are high voltages in the power supply that might not be fully discharged. Since we all seem to have rubber gloves around these days, I'd suggest wearing those when working with the power supplies.
Once you have it all sorted out, you may want to find a reasonably priced 1TB or 2TB drive and install that in (A) -- you'll need to go through the imaging process, but as noted above, the external drives create lots of headaches. Your original drives are also 7+ years old, so once you have the patient on the operating table, you might as well do a drive upgrade. Note, I'd still get it up and running first, as if there is a different (motherboard) problem with (A), you're going to be stuck with an extra drive. Such issues are very uncommon, but there could have always been an electrical issue of some sort or a component failure...
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