Hi, I just upgraded my mid-2012 Retina MacBook Pro from its internal 500GB SSD "card" to a 1TB Samsung EVO 850 mSATA card (using an adapter)...in the past, I had done so with the OWC upgrade kit, without issue, but since this is more of a "home-made" solution, using a readily available and much less expensive mSATA card, it didn't go so smoothly. Long story short, the mSATA card was refusing to be partitioned or formatted until I figured out how to force a different partitioning map (GUID) on the card, which was using MBR, apparently, and was finally able to restore my system partition backup to it...when I reboot now, however, I boot into a black screen. Only when I hold down the Option key do I get the option of booting into either my System or the Recovery partition. My question: is there a way that I can non-destructively convert from my GUID partition map to the Apple Partition Map, so that I can boot automatically, as I was before this upgrade? I know that the gdisk command can do all sorts of magic, and I had to use it extensively to get to this point, so the answer probably lies in a particular gdisk command with the correct parameters to do this conversion. This is where I currently stand: rMBP-Till:~ till$ diskutil list /dev/disk0 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *1.0 TB disk0 1: EFI EFI 209.7 MB disk0s1 2: Apple_HFS macOS-1TB 999.2 GB disk0s2 3: Apple_Boot Recovery HD 650.0 MB disk0s3 rMBP-Till:~ till$ sudo gdisk /dev/disk0 Password: GPT fdisk (gdisk) version 1.0.3 Warning: Devices opened with shared lock will not have their partition table automatically reloaded! Partition table scan: MBR: protective BSD: not present APM: not present GPT: present Found valid GPT with protective MBR; using GPT.