The standard to which you're referring, UFI, supports only 3 media types, all of which are 3.5". It is no use for 5.25" disks. The broader Mass Storage Class is also no use because the built-in OS drivers for MSC assume that the device has a single media type or the hardware can auto-detect the media type. We use a vendor-specific command set so the host can specify parameters to read a variety of formats, including non-PC formats.