[dnanian]

A partition scheme (e.g. GUID, APM, MBR) is not the same as a format (HFS+, FAT32, NTFS). Partitioning divides a 'bare drive' into one or more volumes. Those volumes are then formatted.

Both the partition scheme (which helps the OS and ROM find the volumes) and the volume format (HFS+) need to be correct for a drive to work properly as a startup volume.