Bear with me pls - some stupidity and lots of detail ahead. (Not blog - boot - no edit of title!) I planned to upgrade my MP2008 to El Capitan from Mavericks. I had the upgrade file from when I upgraded my rMBP 2013 to El C. I wasnt able to - as the MP claimed it was corrupted. So after a lot of googling, I decided to move the MP boot disk to a USB3 caddy and try to install by running the installer on the rMBP and installing to the USB3 external. That also failed with same error. So instead of downloading a new copy of El C, decided to try High Sierra. I ran the HS installer on the rMBP, making sure to select the external USB3 / MP hdd. At a point early in the process the machine said it needed to reboot, which I did think was a little odd as I was installing to the external hdd or so i thought. When the process completed i got the select your language screen which I also thought odd. So I declined and shut down. When I rebooted with the external disconnected, i got the flashing folder/?, after i entered my boot/firmware password. So it looks as though the process has overwritten my rMBP boot volume and the copy of Windows id also installed. Things I've tried; Target disc - seems the firmware password stops me from getting to Target DM. I booted the Mac Pro with another disc and attached the external - it shows as a High Sierra volume - so it looks like the install was to the external as I intended. So why my internal rMBP volume has at the same time, been deleted apparently - im completely lost. I could 're-attach the external to the rMBP and complete the High Sierra install and maybe that will allow me access to my files, but I wanted to get some advice here first. I accept there's a fair amount of stupidity laid out there - taking that on the chin Could really do with your help here though. I won't make any more hasty moves until I see what's suggested. It's late here so I won't see all replies until tomorrow. Hope this belongs here rather than El C / High S forums & sorry for lengthy post.