I have a mid 2010 iMac. It's internal HD crapped out. I went to the local Genius bar and learned I could use the Mac with OS loaded onto an external HD. I've done that - loaded OS/High Sierra onto the external HD. I can't get access to the Mac to reselect the external HD over the dead original HD.
How to proceed?
iMac 21.5" 4K, macOS 10.15
On boot, immediately press and hold the option key. This should provide a list of recognized drives, including your external drive. Use the arrow keys to select the external drive, and press return to initiate booting from it.
Once booted from your external drive, visit System Preferences : Startup Disk, and set it to your external drive, if it isn't already. Then you should just be able to power on your iMac with the external drive connected, and boot normally from it.
On boot, immediately press and hold the option key. This should provide a list of recognized drives, including your external drive. Use the arrow keys to select the external drive, and press return to initiate booting from it.
Once booted from your external drive, visit System Preferences : Startup Disk, and set it to your external drive, if it isn't already. Then you should just be able to power on your iMac with the external drive connected, and boot normally from it.
Completely power off your iMac. Provided that a full installation of High Sierra was done on that external drive from a bootable USB stick, the option key on boot solution should detect the external drive.
Press and hold the option key before you even press the power button. No other keys than the option key should be involved. Granted, I have no older Macs here with failed internal hard drives, but the option key is the only key that will present the detected drives for selection.
If paragraph 2 fails, you have a paperweight.
Thanks for your help Viking. This did not work. I first get the spinning globe, then a request for wifi network and then wifi pw. After those steps I get the perpetual and forever spinning turbine graphic. No HD selection. Other ideas?
Dead HD. Wanting to use an external HD to operate OS. . .
