Installing High Sierra on a 2010 iMac
I have a friend who owns a 2010 iMac with a bad drive. I was able to load the OS and run diagnostics under Disk Utility, only to discover that the drive was damaged, as expected. Several attempts to repair the disk were unsuccessful, so I'm now trying to help him recover the apps and data, then format then reformat and reinstall. Hardware & OS is as follows...
Model Name: iMac
Model Identifier: iMac11,2
Processor Name: Intel Core i3
Processor Speed: 3.2 GHz
System Version: OS X 10.10.5 (14F2511)
Kernel Version: Darwin 14.5.0
Unfortunately, my "friend" has NEVER done a system backup, nor recorded App serial numbers and/or license info, etc. (Yes... I am indeed embarrassed to refer to him as "friend") ;-(
My hope was to use the external drive to first make a Time Machine backup, then (as a second safeguard) also manually do a straight file transfer of the data files. There's no guaranty that reformatting the drive will fix the its issues. But this seems to be the greatest chance of recovering as much as possible.
To start, I emptied one of my own older USB drives, then formatted it and installed High Sierra on it (which should be the most current OS available for that model). I know it's a good install, as I can select it as a Startup Drive on one of my own older MBPs which is running under High Sierra. I cannot, however, select this drive (connected via USB) as the Startup Drive on his iMac.
Can anyone suggest a reason why this might be? Do I need to install Yosemite on the external drive (or at least something closer to his current Yosemite install), in order to select the external installation with the iMac?
Any suggestions will help.
Thank you.
iMac