macos – CSM support on 2010 macbook pro with high sierra?

I have a 2010 macbook pro with Mac OS X High Sierra (10.13, and I can’t go any higher). I’m wondering if it has CSM support natively (without installing rEFInd), or if I have to install an alternative boot loader. I know that my mac has EFI support, I’ve used it before. I plugged in an external USB drive with a legacy OS on it (Debian GNU/Linux 10 with grub-pc), pressed the option key while the computer was starting up, and the USB drive did not show up in the boot manager. I detached and reattached the drive but again, nothing showed up.

Please let me know if I have to install some software to make ths work.
Thanks, Lennon