UEFI

Say you have an drive on a Linux computer that was created in the days before UEFI was widespread. And say, you want to move it to a more modern hardware. In this case, you may want to convert that drive from Legacy BIOS to UEFI.

This is not an easy process, but quite doable, though it is a multi-step one. There are several guides online, like this one, which details the process.