Lenovo ThinkCentre M91p dual-boot only boots Windows

I tried dual booting Windows 10 & EndeavorOS on this system but it will only boot windows and not recognize grub. I then installed EndeavorOS alone and it still does not recognize the OS. I went ahead and tried a Debian based OS (Kubuntu) with the same results. Is anyone familiar with this hardware? Is it possible to get Linux to work on this hardware? Appreciate feedback. Thanks.

Lenovo have pretty good support for linux so I would not expect it is a hardware problem. Have you ever created a dualboot on a different hardware? There are a few things that has to be configured so that windows doesn’t act so aggressively.
In BIOS check if you have secure boot enabled. If you don’t need it then I recommend to disable it. Otherwise it is little bit complicated to add a signed kernel.
Next in windows under power management there should be an option for “fast boot” (I do no know how exactly it is called). That has to be also disabled. When you do that you should be able to ented grub at the next boot (you may have to select the proper boot option at the next startup).

Under Windows that feature is called Windows fast Start Up.

Thanks for the feedback. There is no secure boot option in the bios. You can choose between Legacy, UEFI or Auto. I turned off fastboot in Windows but it did not make a difference. There is some discussion on the web about this issue with this computer line some years ago indicating it is hard coded to Windows in the bios. I have dual booted many times with other computers without this issue. I decided to just install EndeavorOS which boots as long I set the bios to Legacy mode… and it works great. I will. just run Windows in a VM. I spent enough time already and it is just not worth it. Much appreciate your time on the matter.

You might accidentally have installed EnOS in Legacy mode while your Windows is in UEFI.

I would set the BIOS to UEFI only and reinstall EnOS in the same mode.

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and what is this set to?

I would double check BIOS/firmware settings. There must be a setting where to set which OS or disk to boot. It may be hidden under some admin password somehow, but that’s up to the machine specifications.
And there should be a secure boot setting when using UEFI mode.

In which mode (UEFI or BIOS legacy) is Windows installed? Install EndeavourOS with the same mode.

And have you updated the BIOS/firmware to the latest available version?

Also, have you enabled a RAID setup on your disk?