Greetings! Finally took the plunge on upgrading my late 2012 Mini, and installed a second 1TB SSD. I still have the original HDD installed, which was the start-up disk – and now, despite my best efforts, cannot get to erase it! I want to leave the OS/start-up on the SSD, and keep the 1TB HDD as hard storage, with both being backed up via TM to a DAS. Background: Have Server running on the HDD, with Sierra (latest) installed. Been having issues upgrading to H-Sierra, but guess that's a story for a different thread. Plugged the new SSD into the Mini as a external, fired up Recovery Mode, and installed macOS Sierra onto it. Then used File Transfer assistant(?) to get everything off my HDD, and over onto the SSD. When that was done, installed the SSD inside the Mini, and then started her up holding Option down, and booted into the SSD. Checked all was as it should be, and confirmed that the SSD is the start disk, inside System Preferences. If I reboot, boots onto/into the SSD - so all ready and working as expected. Now, move on to erasing the HDD. Open up Disk Utility, select the HDD and "erase" - error, cannot erase the disk as it failed to unmount. Right - do some reading, seems this can happen if something is trying to modify the disk - figure it might be a TM backup or something. So advice is to restart, and boot into Recovery Mode. Reboot, hold down Cmd+R >>> and... chime.... and... starts booting into the HDD. I know, since the progress bar is waaaaay slower to start rolling, unlike the new SSD. Regardless, in Recovery Mode - go into Disk Utility, and try and erase. Same story >> cannot unmount disk. Over on the Apple logo, select Startup disk, and confirm the SSD is selected - reboot again. Hold down CMD+R >>> and... chime.... and... starts booting into the HDD!! This keeps happening. I cannot seem to boot into Recovery using the SSD. So presumably I cannot erase it, because it cannot unmount, because it keeps booting into the HDD when I try and launch Recovery Mode! What on earth? How do I force it to recovery-mode boot into the SSD, so that I can nuke the HDD? Can this really be so hard? Is Server playing some role here - is it defaulting to the HDD since it thinks of that drive as the "main drive", regardless of what I try and boot from? Any suggestions would be greatly appreciated!