My HP Envy laptop boots netbsd in both efi and bios modes, from internal sata disk and external usb one. Both modes are enabled simultaneously. My default efi boot is rEFInd, works fine for me. I installed NetBSD-current on some free space at the end of a gpt disk which already contained a couple of Linux distributions and some Windows data, following the outline described above. rEFInd also automatically recognises other systems on external disks and can boot them (usually; in some cases this didn't work, e.g. with a Fedora on some external mbr disk, in which case I had to get in front of rEFInd with F9 and choose the disk directly).