Will the install fail, or will my iMac just not support all of High Sierra's features.
I don't expect to get full functionality, but I do want to keep my iMac running. TurboTax already warned me that it won't support my OS next year. OS X 10.11.6 has been running fine for almost 2 years.
iMac, OS X El Capitan (10.11.6), 2x3.06GHz / 8GB DDR3 / 1TB HDD
Can I Update to High Sierra from El Capitan?
The answer is no. Excerpted from How to download macOS High Sierra - Apple Support:
Can your Mac use High Sierra?
High Sierra requires any of the following Mac models ...
- iMac models from late 2009 or later
- ...
Can I Update to High Sierra from El Capitan?
The answer is no. Excerpted from How to download macOS High Sierra - Apple Support:
Can your Mac use High Sierra?
High Sierra requires any of the following Mac models ...
- iMac models from late 2009 or later
- ...
Will the install fail, or will my iMac just not support all of High Sierra's features.
No, it shouldn't even download, because your iMac can't run anything beyond El Capitan.
See App Compatibility Table - RoaringApps. For upgrading, macOS see How to download macOS High Sierra.
Early 2009 iMac: Can I Update to High Sierra from El Capitan?
