There is no direct software upgrade path to mac OS High Sierra (10.13.x) for an iMac (21.5-inch, Mid 2011). Besides that, major updates usually have BOOT ROM / SMC action involved. Can’t miss those! 🙂
Internet Recovery managed to grab OSX Lion (10.7), updated to the latest then my backup copy of Mountain Lion (10.8) to continue. Unfortunately, the App Store dropped an error every time I attempted to install Update 10.8.5 combo. The workaround was downloading Updates 10.8.1 through 10.8.5 and manually installing them one at a time.
After 10.8.5 installed, I ran Update using the App Store to get the latest 10.8.x updates but this time around, I didn’t encounter any errors.
Continued to run through mac OS Sierra (10.12) installer and rebooting several times to fetch the latest updates.
Finally, I hooked up a drive with an existing fully patched High Sierra (10.13.6) system using a SATA-to-USB cable (with power), rebooted and used it to format (APFS) the built-in 500GB harddrive on the iMac before dumping a fresh installation of High Sierra.
Just for fun, I booted off a drive with an existing fully patched Windows 10 (version 1909) system without using BootCamp Assistant. It worked as expected.
In the end, my trusty #FreeBSD server running off an old Dell Dimension 518 was a big help giving me access to storage/files i needed.
I’ve said this before and I’ll say it again, Apache/WebDAV still works. 🙂