I'm hoping somebody here knows more about bootloaders and such than I know.

Here's the situation...

I bought a cheap computer to turn into an open-source router. It came with Linux Mint installed. I installed another Linux distro via a USB flash drive just to try it out. When I tried using the same flash drive to install the router software the computer won't recognize the USB to boot from it. I know the USB is functional because it works on this laptop when I try it. And I know the USB ports on the other computer are good because I just installed the other OS a few hours earlier through those same ports. But now when I tell BIOS to boot from the USB it skips straight to the HD.

I have gotten into the GRUB terminal and run "USB" and it shows no connected devices. I'm assuming that means the ports aren't mounted or something. I was going to do further diagnostics through the OS I just installed, but now it won't boot cleanly either. So I can only get into either the GRUB terminal or a stripped-down version of the OS terminal.

How can I get the computer to boot from the USB again?

Also, I did set the bootable flag on the partition when I reformatted the USB stick.

When you have an USB connected external hard drive, does it show up in BIOS?

Yeah, BIOS sees the stick. I think I've diagnosed the problem; the distro I'm trying to install uses EFI. The old HP I'm trying to install to is from way before EFI was a thing. So the BIOS has no idea what it's seeing. I might as well be sticking a carrot in the port.

I hope I can find a way to reconcile this incompatibility. I'm trying to install OPNsense to turn this old box into a nice router/firewall. I don't have a huge selection of options if this won't work.

I did try using fdisk and gdisk to get rid of the GPT and replace it with an MBR. But that didn't help. I'm sure I didn't do it correctly though. The documentation on the process is pretty sparse and mostly centered around converting in the other direction.