Debian 10 on Lenovo not shutting down

Hi,

I have an old "lenovo m55e 9644" with debian 10 installed. I want to use it to backup some other systems.
I don't want it to stay on 24/7 though, I want it to turn itself on (or maybe via WOL), take a snapshot of the other systems and then shut itself down.

The issue I have is that instead of shutting down, it keeps rebooting itself and I don't understand why.

All I know is that this only happens when there is a USB key plugged in.
I can't remove the USB keys as that's where the OS is installed (mainly due to the low number of SATA ports).

At first I thought this might be caused by something in the BIOS so I disabled/changed everything I could, like:
Advanced: Legacy free
- changed from "disabled" to "enabled" (no difference)
Power: PCI Wake Up
- changed from "primary" to "disabled" (no difference)
Startup: Automatic Startup Sequence
- changed from "enabled" to "once" (no difference)
Advanced: Plug and Play OS
- changed from "no" to "yes" (no difference)
Power: ACPI BIOS RQ
- changed from "9" to "x11" (no difference)

I normally use "shutdown -h now" and what happens is that the power light goes off for just about a second, and then the machine turn itself back on.

Any idea of what I could check to debug this from the software side, please?
For example I wonder, which part of the OS would be making the machine reboot?

Regards,
Andrea