How to fix dual-boot issues on iMac with macOS Mojave?

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Level 1

36 points

How to fix dual-boot issues on iMac with macOS Mojave?

Hi everyone, I need some help please to fix my issue with our 2017 iMac 27" Retina 5K, Intel Quad-Core i5 3.4GHz / 32 GB RAM / 3TB Fusion Drive.

We needed it to run OS Mojave so we can keep using Adobe's Creative suite and not go to their expensive subscription model. The iMac itself is great but using old browsers was causing issues with modern websites. I bought a 2TB SSD, connected it to a USB port and loaded OS Ventura onto it. We have been using that as the startup disk for the last week and can access all our files, docs etc without any issues, however... I cannot get the iMac to boot from the original internal drive, so we can't get back to the Adobe software we need.

I have tried

1) From power off, hold the option key and power on, select the original hard drive.

2) From OS Ventura, selecting the original startup drive in System Preferences as the Startup drive.

3) Re-installing OS Mojave to the original drive from a USB thumb drive (which I have used many times before).

In all cases the machine starts to boot, the white progress bar moves across the screen and then everything stops. The progress bar is fully across but there is a also a white circle with a diagonal line through it.

I have left the machine in this state for up to 1 hour hoping it would complete the boot, but it never does.

What am I missing or where else should I be looking to get back to the original OS? We do have TM backups right up to the day before I installed Ventura but I am thinking that re-installing from TM might corrupt all the files that are now configured for Ventura.

Thoughts and suggestions please :-)

John

[Re-Titled by Moderator]

Original Title: Dual-Boot problem

iMac (2017 – 2020)

Posted on Jul 30, 2025 4:47 AM

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Answer 1

VulcanJohn Author

Level 1

36 points

Jul 31, 2025 5:42 AM in response to VulcanJohn

Thanks for the suggestions. I can now breathe a BIG sigh of relief having brought the original drive back to life with Mojave running and all the Adobe applications working, no issues with activation codes etc.

After trying a different external drive with Mojave, which again wouldn't boot, I took the scary step of doing a full restore from the last TM backup before Ventura came on the scene. Why scary? Because part of the restore is to totally erase everything on the installed drive, so if TM didn't work then all was lost.

After 10 hours the machine rebooted, asked for our Apple account details and reloaded all iMessages, Mails and the other assorted stuff we load our machines with. Lesson learned, if it ain't broke, don't fix it! So, we will be staying with Mojave on the iMac and using newer iPads etc for any web browsing where sites don't like the older browser. As for Adobe, they can keep their subscription model, never going there. As a retired couple with infrequent use of the CS6 suite there is no way we can justify the exorbitant cost of the Cloud versions. Take care with what you mess with and ALWAYS have a backup!

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Answer 2

Jul 30, 2025 6:54 AM in response to VulcanJohn

Sounds like the internal drive is broken. So you can't erase and install from an USB Mojave installer to it, right? Can you erase and mount it as a data drive at all? Can you install Mojave to an external drive and use Time Machine for the restore?

I use Sequoia but I can also dual boot Mac mini 2018 to Mojave to run old 32-bit apps like Photoshop CS6 13.0.0, Lightroom 6.14, MPEG Streamclip 1.9.3b8, QuickTime Player Pro 7 etc. But I have found Sequoia compatible alternatives so I rarely do that. My son got tired of the 60 seconds it takes to dual boot and bought Photoshop and Lightroom subscription so I use that now.

Beware that re-installing those old Adobe apps from scratch might be difficult because Adobe has shut down its license servers or otherwise made the task demanding.

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Answer 3

Jul 30, 2025 8:18 AM in response to VulcanJohn

You might consider installing Mojave on another external drive and booting from that, just as you are booting Ventura. If the internal drive is failing then this would be the workaround and allow you to continue to use Mojave.

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