"Missing a firmware partition" iMac running Capitan off external HD

[stevedawg85]

Update Subject: Missing a firmware partition" iMac running Capitan off external HD unable to upgrade to Mojave


iMac late 2013
10.11.6 El Capitan

Long story short, bought this machine on the cheap from Craigslist, but internal HD was dead. I did not want to replace the HD, I instead installed and running the HD off a 3.0 external HD. It works great, but now I want to do an upgrade, and am getting the error when attempting to update to Mojave:
"Missing a firmware partition"

Any way to workaround that? I still do not want to open it up.

 

[Fishrrman]

Have you tried erasing the internal drive with Disk Utility?
(I will guess that you already have)

Does your external drive have a recovery partition on it? If it doesn't, I'm wondering if not having one could produce such an error.

Have you tried upgrading to either Low Sierra or High Sierra first?
I'd suggest either of them, because Mojave is still very much "a work in progress".
High Sierra has been updated "as far as it will go". I'd reckon all the bugs have been "worked out of it" by now...

 

[mikehalloran]

The problem is that you have a 2011 and the installer won't work by design.

Mojave requires a Metal Graphics compatible GPU. In short, a 2012 or later if an iMac.

Not saying it isn't possible to install. I have done so onto an external drive and tried that on both my 2010 & 2011. 'Tain't pretty... or practical.. in fact it's kinda' useless but it can be done. There's a hack and a patch. It works.
http://dosdude1.com/mojave/

Do pay attention because this applies to you: "Currently, it is not possible to get full graphics acceleration when running Mojave on a system with a Radeon HD 5xxx or 6xxx series GPU. Mojave will be almost UNUSABLE without graphics acceleration." Oh my yes. I did it to see which apps I have to upgrade (a few only) and to find out how compatible my database was — better than High Sierra.

BTW, the 2011 is one of the easiest iMacs to crack open and upgrade. Takes me a half hour one-handed (seriously, I've a crippled arm and have done a bunch of these).

I, OTOH, am now in the market for a later iMac. My wife has no need for Mojave so her 2011 is fine.

 

[stevedawg85]

Thanks for the replies. I'll try the High Sierra option instead and see if I have any luck. Maybe it is time for a hardware upgrade, which would mean I can maybe open it up and try to fix it. Thanks for the inspiration Mike!