Question Why Won't PC Recognize New HDD?

Oct 5, 2021
I have a Dell Optiplex 390.

I have an HDD with Windows 10 installed but when I connect the SATA and Power Cables to the HDD, all I see is a little white line up in the top left corner blinking with no response from the keyboard or mouse.

I have started and restarted my computer but I have yet to see windows on start up. I am wondering if this is an MBR/GPT and Legacy/UEFI issue?

I think the HDD has GPT partitioning but the system was set in Legacy mode when the HDD was first installed. Could this cause the computer to change the partitioning from GPT to MBR?

In the BIOS screen, I don't see MBR. All I see is the hard drive is in Legacy mode but when I switch to UEFI I don't see the hard drive.