The front panel menu system:

Global->About->BSP-> Linux 4.9.47-rt37 2019-12-20.C23818C

Granted, I cannot prove that because it says the above that it's really running Linux, but I'd guess it is.

> Also, do you know which microprocessor(s)/FPGA(s) the Plethora X5 uses?
>

No idea at this time. Also no idea about A/D & D/A.

In my mind the unit is essentially a 1 channel DAW that can handle 5 plugins as well as an insert bus that can be put anywhere in the chain. Running plugin modules, be they LADSPA or whatever is more or less how I see the design.

> I'm partly asking because I wonder if the X5 might be able to run
> software from Mod Devices or vice versa.  Mod Devices makes ARM-based
> multi-FX guitar pedals that run Linux: akin to the X5 but using
> LADSPA/LV2 plugins instead of TonePrint ones: https://moddevices.com
>
> Finally, do you know if TC Electronic has published any of the source
> code relevant to the Plethora X5?

<SNIP>

I haven't looked hard for any of that but I also haven't seen it in the manual or on their website.

<SNIP>

> If you plug the Plethora X5 into a Linux box via USB, how does it
> present itself to the host?
>
>     # Run this on your GNU/Linux box with the X5 not connected:
>     lsusb --tree > /tmp/lsusb_x5_disconnected.txt
>
>     # Run this on your GNU/Linux with your X5 powered up and connected
>     # to it via USB but everything else the same as above:
>     lsusb --tree > /tmp/lsusb_x5_connected.txt
>
>     # Copy the output of this command and paste it into a reply to this
>     # email:
>     diff /tmp/lsusb_x5_disconnected.txt /tmp/lsusb_x5_connected.txt
>

<SNIP>

A first response from TC Electronic via a moderator named 'TC-Mike' is that the ability to backup the device is 'on the roadmap'. 

I asked about general timeframe but have received no response.

Cheers,

Mark