OS X Mojave (10.14.* Mojave) has a surprise waiting for you ...

[DelJo63]

Prior systems were implemented on the HFS+ journaling file system.

Mojave and beyond (Catalina) requires AFS and will attempt to non-destructively convert on the fly. Various conditions will cause the conversion to fail and all of this effort will be for not! Several online sites note that starting in recovery mode can get you to the Disk Utilities and that the
Edit->Convert to AFS is available; DID NOT WORK FOR ME (on High Sierria) and that choice was grayed out.

High Sierria reaches EoL at the end of Nov, 2020 so this is non-trivial

Do your homework and by all means, get a backup first -- Caveat Emptor.

 

[DelJo63]

The hidden requirement is:

Machines that ARE NOT supported:​

​

• 2006-2007 Mac Pros, iMacs, MacBook Pros, and Mac Minis:​

• MacPro1,1​

• MacPro2,1​

• iMac4,1​

• iMac5,x​

• iMac6,1​

• iMac7,1​

• MacBookPro1,1​

• MacBookPro2,x​

• MacBookPro3,1​

• Macmini1,1​

• Macmini2,1​

and my High Sierria is on
open terminal and enter:
system_profiler SPHardwareDataType

Hardware:​

​

Hardware Overview:​

​

Model Name: iMac​

Model Identifier: iMac14,1​

Processor Name: Intel Core i5​

Processor Speed: 2.7 GHz​

Number of Processors: 1​

Total Number of Cores: 4​

L2 Cache (per Core): 256 KB​

L3 Cache: 4 MB​

Memory: 8 GB​

Boot ROM Version: 146.0.0.0.0​

SMC Version (system): 2.14f24​

​

SO, to remain on a supported system, it's off to the Apple Store with a bag of money :sigh:

 

[SNGX1275]

Well, to be fair that Mac is over 14 years old.

 

[DelJo63]

It's a iMac (21.5-inch, Late 2013) so it's only seven years old. Regardless, at EoL the Mac forces a hardward change, which in my mind is questionable. Going non-support is fine and to be expected.

 

[SNGX1275]

Ah, my apologies, that 4,1 highlighted initially is what spurred my comment. I didn't think too hard on it, but a 2006 intel mac wouldn't be running High Sierra anyway.

 

[DelJo63]

No biggie ; Those codes are anything BUT obvious in the first place.
That system started as Yosemite (10.10.*) and has been moving forward up to Mojave and that looks like a brick wall.