I followed the instructions given in the linked answer and was not able to reproduce the problem described in the posted question. The Windows version and flash drive used are given below.
- OS: Windows 11 Pro (Version 22H2)
- Flash Drive: Micro Center USB3.0 16GB
When holding down the Option key at startup, the flash drive appeared on the Mac Startup Manager menu with the given label and icon for the following test Macs. In all three cases, the Mac was able to successfully boot from the flash drive.
| Mac | Identifier | Label | Icon |
|---|---|---|---|
| iMac (20-inch, Mid 2007) | iMac7,1 | Install El Capitan | Default External Drive |
| iMac (21.5-inch, Mid 2011) | iMac12,1 | Install El Capitan | Default External Drive |
| iMac (21.5-inch, Late 2013) | iMac14,3 | Install El Capitan | Image from .VolumeIcon.icns |
I did encounter the following issue when using the 2013 iMac to test. The flash drive did not appear on the Mac Startup Manager when another bootable USB SSD was also plugged in.
Alternative Solution
Try using Windows to create a Mountain Lion USB installer. The instructions are much simpler.
Note: I did not need to change the Apple Partition Map (APM) to a GUID Partition Table (GPT).
When holding down the Option key at startup, the flash drive appeared on the Mac Startup Manager menu with the given label and icon for the following test Macs. The 2007 and 2011 iMacs were able to successfully boot from the flash drive. The 2013 iMac would not boot and halted with a prohibited sign showing on the screen. I assume the 2013 iMac is new enough to be incompatible with Mountain Lion release provided by Apple.
| Mac | Identifier | Label | Icon |
|---|---|---|---|
| iMac (20-inch, Mid 2007) | iMac7,1 | Mac OS X | Default External Drive |
| iMac (21.5-inch, Mid 2011) | iMac12,1 | Mac OS X | Default External Drive |
| iMac (21.5-inch, Late 2013) | iMac14,3 | Mac OS X | Default External Drive |
Once Mountain Lion is installed on your Mac, you could then upgrade to El Capitan.