How can I make a new UEFI-install computer work with a Legacy-install HDD?

[Alex12121]

I have purchased a Victus HP TG02-0823nw desktop ( https://support.hp.com/si-en/document/c08215598 ) and before purchasing it, I was assured at two different stores that I would be able to simply connect my old/current HDD with my Windows install into the computer and upon boot it would offer me a choice of which system to load up, so that I wouldn't have to reinstall all my programs again. Upon connecting my HDD to the computer all that happens is it comes up with a blue box reading "Boot device not found. Please install an operating system on your hard disk", despite the HDD having a working install of Windows 10 currently on it and the Windows 11 install SSD is also connected. The same message appears when removing the SSD and trying to boot up the desktop with only the HDD connected.

Apparently it might be an issue that my Win10 is installed in Legacy mode and not UEFI. How can I make the desktop work with my Legacy-install HDD? It's not even recognizing the HDD as an additional internal drive and not allowing me to boot up the machine using the Win11 install that is otherwise working fine on the new desktop to even just have access to my programs or data.

[WAWood]

When you boot, the computer is either in Legacy mode or UEFI mode.  The mode has to match the Windows installation.  So, if you have two different installations, one in each mode, you have to keep manually switching the boot up mode to match.

MS provides a utility to switch an MBR-formatted drive to GPT-formatted, and vice-versa -- but that just changes the drive formatting, it does not change the Window boot from Legacy to UEFI.

[Alex12121]

How do I switch the boot mode to Legacy? The UEFI install has nothing but Windows 11 on it, but as soon as I connect my Windows 10 HDD to the computer it doesn't even boot up, only gives me an error that I need to install an operating system, even though it already is installed. I can get to the BIOS but otherwise it's as if the desktop is disregarding the OS it came with.

[WAWood]

The UEFI code only sees what it connected at the time.  If you want to see the Win10 drive in the UEFI boot menu, the drive has to already be connected when you boot the PC.  But, if Win10 is set to boot in Legacy and has MBR-formatted drive, not GPT-formatted, I'm not sure the UEFI will even see it.

[Alex12121]

Apologies for slow replies and thank you for your assistance. I do try to boot the desktop with the Legacy HDD connected but it only gives me an error that there is no operating system installed, even though the SSD with the Win11 install is still connected and should be read and launched, I don't understand why it does that as even if it doesn't read the HDD install it should still default to the SSD install that the machine came with and that it loads up just fine otherwise?

Would my best bet at this point be to try and reformat my HDD into GPT (if that is even possible) and hope that my files will be okay and then the desktop can actually see it and cooperate?

[WAWood]

As I said, you can only boot in one mode or the other.  If the SSD is UEFI/GPT and the HDD is BIOS/MBR, you would have to switch boot modes when switching drives.  You will not be able to see both drives in either mode.