Operating system loader failed signature verification

I’m sure there are lots of posts out there, that point to info on this… PLEASE: Point me in the right direction!

My PC cr#pped itself, so led me to purchasing a new (to me), PC.
Old PC: Optiplex 9020 Mini Tower… Not Win11 compliant.
New PC: Optiplex 9060 SFF. Win11 Compliant
So, decided to try putting old HDDx2 (GPT partitions) into new Computer - they are OK.
HDD#1: Boot drive with Win10/Tumbleweed/Ubuntu
HDD#2: Various ‘inactive’ partitions.

When I try to boot the old drives, Win10 boots fine, but Linux gives:
“Operating system loader failed signature verification. Warning: The file may have been tampered with!”

Help!
Can I fix this? How? Or do I have to reinstall? Tried running install USB, but get the same message…
Thanks,
John.

Do you boot with secureboot enabled? And if so will the problem occur with secureboot disabled as well?

Sorry, yes I was over(under?)thinking the whole thing…
Realized eventually that Secure Boot WAS selected (was sure I had disabled it…)
But, the whole booting with UEFI, is not very straight forward - have to go in and figure out what you need to boot, in a text file…? ie, had to go through trial and error, to find the correct setting to boot the Grub file…
Gotta get up to speed on this… (Ummm, catch up??)
Thanks!

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