Help with 2009 iMac to install High Sierra

[CooperBox]

I was recently gifted a late 2009 21" iMac which functions perfectly with OSX 10.6.8 Snow Leopard installed on the original HDD.
I've installed High Sierra 10.13 onto an SSD (formatted APFS) which I plan to install into the iMac.
Using an external hard drive enclosure I've tested the new High Sierra SSD with my 2012 MBPro (running Mojave) and it boots without problems. However when I try the same with the SSD connected to the iMac by USB it doesn't appear on the desktop. Tried 2 of the other USB inputs but still no desktop indication. Then checked with disk utility which shows the presence of the SSD but with all the disk utilty actions greyed-out.
If I installed the SSD inside the iMac is it likely to function correctly or is some later firmware required to upgrade from Snow Leopard to High Sierra? Any clarification or ideas will be most appreciated.

 

[CoastalOR]

See Technical Specifications for High Sierra;

macOS High Sierra - Technical Specifications - Apple Support

support.apple.com

I think since you have not installed 10.8 or greater the firmware can not run High Sierra and not even recognize a APFS formated drive.

 

[macnavarra]

Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter A1790 + TB2 2M Cable, connect direct to another Mac, boot the other Mac with a macOS usb boot drive, disk utility to partition etc.., reinstall or install another supported macOS version and at a given time use the screen dialog to copy your "settings".

 

[Adora]

The installer to 10.8 can be found here and after updating, High Sierra should work from the App Store and all Firmware updates will be done, when you do both upgrades.

https://updates.cdn-apple.com/2021/macos/031-0627-20210614-90D11F33-1A65-42DD-BBEA-E1D9F43A6B3F/InstallMacOSX.dmg