You cannot change that setting without either running a conversion utility or reinstalling Windows.

Before running the conversion utility, make and verify a complete system backup and recovery media -- things do go wrong in the process.

https://docs.microsoft.com/en-us/windows-server/storage/disk-management/change-an-mbr-disk-into-a-gpt-disk

@ejn63  That is the wrong set of instructions for this scenario. The correct link is below, which involves the MBR2GPT utility that not only converts the partition layout scheme but also creates the additional partitions necessary to allow a disk containing a Windows installation to be booted in UEFI mode. The steps in your article are intended for data disks and involve deleting all existing partitions. But even if it weren’t for that, a basic MBR to GPT conversion would leave the system unbootable when performed against a disk that contained a Windows installation.

https://docs.microsoft.com/en-us/windows/deployment/mbr-to-gpt

But @lam3001  why do you think you need to disable Legacy Option ROMs to enable TPM? That is not a general requirement since TPM existed long before UEFI boot mode (which is all you can use if you disable Legacy Option ROMs). I guess there might be something specific to this system, but I think it’s more likely that someone just didn’t know what they were talking about.

If you want an even easier way to get Windows 11 installed, look at this Microsoft page, specifically the “Other ways to install” section. It describes a registry tweak you can apply that will cause Windows 11 to bypass the CPU and TPM requirements, so you won’t have to reconfigure anything.

@lam3001  Ok in that case that option is keeping Legacy BIOS boot support enabled even though you don’t need it. And that is also preventing you from enabling UEFI Secure Boot, which you should use if you’re running Windows as opposed to some Linux OS that doesn’t support that. So go ahead and disable Legacy Option ROMs and then enable UEFI Secure Boot. I don’t think that will fix your TPM issue, but it’s a good thing to do anyway. And worst case you can always change those settings back. You won’t permanently damage anything — although I fully expect you to be completely fine after making those changes.

FYI I reviewed this and my drives are already GPT.

Hi, “Firmware TPM” is currently Disabled and greyed out in the UEFI/BIOS settings. I cannot change it. I found this article indicating I need to disable Legacy Option ROMs first:

Article

Will it also let me turn on BitLocker? Separate issue really from the Windows 11 TPM requirement but also something I have been wanting to do.

@lam3001  That's not exactly an article.  That's just another forum post by another person.  But if this system has a virtual TPM implemented in firmware, not a physical TPM chip, then I can see why you might have to run the system in pure UEFI mode.  The changes I described will achieve that and should therefore allow you to enable that option, if that other person in that thread you linked is correct.  And then yes that should then also allow you to enable BitLocker if you're running Windows 10 Pro.  If you're running Windows 10 Home, then support for encryption is more limited, but it may still be possible for your Windows partition (but not other partitions/devices), since Windows 10 Home does have support for "Device encryption", which is a more limited version of BitLocker.

@jphughan​ I know this is an old thread, but I have an inspiron 17R SE 7720 and I HAD to leave the load legacy option rom setting in the bios enabled.

The machine has win 10, and I had changed from raid to ahci, then ran mbr2gpt, then set the boot to uefi. This all worked fine.  I was playing around with the bios settings and when I disabled the load legacy option rom, there was no longer any way to boot a usb flash drive using f12.  With the legacy rom enabled,  f12 gets the list of legacy boot options as well as uefi windows boot menu. Oddly enough, selecting usb drive from the legacy list not only boots the usb stick, but it actually boots it into UEFI mode. I can tell because a macrium rescue usb shows uefi in the title bar.

I thought this might be worth mentioning in case others come across this old thread.