Watchguard device serial number/BIOS compatibility

[zanthos]

Before attemting to upgrade my existing x750e, I need to know where the device serial number is stored.
Is it in the bios chip? So I better keep a backup before upgrading to 8.1…
Is there any other memory to store the serial number?

I know the serial number is not stored on the CF (Watchguard OS).
(I tried to swap another x750e's CF to this unit and it showed me wrong Watchguard licenses due to a diffrent serial number).

I just want to make sure, if for any reason I'll need to switch back to Watchguard OS (some Linux...), it should be working again!

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Would the 8.1 BIOS work with the original Watchguard OS?

[stephenw10]

I am almost certain that it isn't stored on the BIOS ROM. However, yes, make a backup first.
I have never attempted to go back to the Watchguard OS personally.

Steve