I've learned that my machine contains SeaBIOS, which is legacy-only and doesn't support UEFI. But drives >2TB need to be formatted in GPT which requires UEFI for the BIOS to read all sectors/blocks. At least, I believe this is accurate. But the posts where I read about this were focused on booting from the drive. I'm not doing that-- I want to boot from the internal 32GB drive and use a large >2TB external storage drive.