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iMac no longer recognizes USB installer

torbarr

macrumors newbie
Original poster
Aug 14, 2020
2
0
Hi folks, my 2019 5K iMac (which came with Mojave from factory) doesn't recognize when booting (with alt key pressed to access the boot options) a USB installer of Mojave and backwards (El Capitan etc), they literally don't show up when boot options appear. Funny thing is, it ONLY recognizes a Catalina USB installer.

The Mojave USB installer is fine (created via createinstallmedia, formatted GUID and MacOS Plus), it DOES show up in two MacBooks (plus it always appeared in the iMac last year when I did a clean install of Mojave, the same USB stick). Curious thing is, when I enter the existing installed OS to properly shut down the computer after invoking boot options (with Alt key pressed right after turning on the iMac), the USB stick does not appear nor mounted or even in Disk Utility side bar; it literally doesn't exist. Surprise comes when I manually take the USB stick out of the iMac and then stick it again while logged in and voilá!, it appears normally, and if then I restart (not turn off and on the computer) it does show up in the boot options.

Anyways, I post a timeline so it'll be easier to explain and understand:

2019 07 -- bought iMac with Mojave preinstalled. Used it normally.

2019 10 -- reinstalled Mojave clean from USB installer made with createinstallmedia tool. Used it normally.

2020 07 -- changed iMac's PSU in an official support service after issues with it.

2020 08 (1) -- tried to reinstall Mojave from a USB installer because I had to provide my iMac user's password in the official support service, and I wanted a clean install with new passwords and new all (y' know). BUT, problem comes here, the iMac won't recognize the new USB installer of Mojave at boot, not even when I log in the current still installed Mojave: it doesn't show even at Disk Utility! This USB installer DOES show up at boot in other Macs though.

2020 08 (2) -- I unplug the USB and replug it in the same user session, it is, with the iMac on and logged in, and magically the USB Mojave installer is mounted and appears on the desktop!

2020 08 (3) -- Just in case I re-create the Mojave USB installer, with other USB drives and attached in different USB ports of the iMac, but this doesn't seem to work either.

2020 08 (4) -- Just for testing purposes, I plug in an old El Capitan USB installer I had somewhere else and same result: doesn't show up in boot options. Plug it in the other Macs, and it shows up flawlessly.

2020 08 (5) -- To test out, I plug a newly created Catalina USB installer and this DOES appear in the iMac at boot. Don't know why the Catalina's USB appears and the others don't.

2020 08 (6) -- Because perhaps it is some EFI firmware bug present in the iMac, I clean install Catalina so it has the up-to-date firmware version of the EFI. During install, the EFI gets updated as expected. But, the boot issue with the Mojave USB installer persists.

At this point I don't know what more to do or test. Can't install Mojave from the USB and because I tried Catalina in another Mac and my experience was awful, I am trying to avoid Catalina at all costs.

My bet is that it is an EFI/Boot ROM firmware bug that didn't exist before and persists through various firmware versions since then :S Would be really interesting though to know if someone has already experienced this same problem!

Would also thank any kind of suggestions. Cheers!
 

chrfr

macrumors G4
Jul 11, 2009
10,192
3,909
2020 08 (1) -- tried to reinstall Mojave from a USB installer because I had to provide my iMac user's password in the official support service, and I wanted a clean install with new passwords and new all (y' know). BUT, problem comes here, the iMac won't recognize the new USB installer of Mojave at boot, not even when I log in the current still installed Mojave: it doesn't show even at Disk Utility! This USB installer DOES show up at boot in other Macs though.
Are you sure that your Mojave installer is of 10.14.5 or later? The iMac can't boot from anything older than its special version of 10.14.4, so it's unsurprising that older installers don't show.
You can always boot using command-option-shift-R to use internet recovery to reinstall Mojave.
 
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torbarr

macrumors newbie
Original poster
Aug 14, 2020
2
0
Hi chrfr! Thanks very much for answering, I cannot find any info about this concrete behavior anywhere :S

Yeah, I've tried too with the latest Mojave version available now, the one with the updated certificates, but no luck.

I am aware of the Internet recovery tool by pressing Cmd+Opt+Shift+R, thanks anyway for saying. I guess I'll have to use it in order to install Mojave, but still on a curious level I'm really intrigued about the real reason of this strange behavior... As said, the Mojave USB installer was recognized flawlessly at boot the last time I reinstalled Mojave in the iMac last year...

Cheers!
 
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