iMac not starting up, grey screen, question mark folder, no access. Help please

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Level 1

39 points

iMac not starting up, grey screen, question mark folder, no access. Help please

Hi,

Earlier this year, I installed a new 1TB SSD into my iMac to replace my 3TB fusion drive which had died. It turned out the new drive was too small to restore from backup because I had so much stuff on it so I ended up just installing an external SSD...also 3TB, which handled everything fine.

It's been working OK, but sometimes it just shuts down randomly (not often thankfully)...occasionally when this happens there are problems starting it up....in Disk Utility the start up disk can't be found...but this is not always the case....after a couple of attempts the mac usually eventually finds the right disk and all is well.

Today however, I decided to shut down the imac as i'd had it open for weeks without giving it a rest. Unfortunately for a few hours I've not been able to start it up again. I get the grey screen with a flashing folder with a question mark which lasts 15-20 seconds, then a 'no entry' type sign for 15-20 seconds and then we're back to the 'prohibited' icon...and this alternation continues. So, it looks like the startup disk can't be found.

When i go into Disk Utility, however, it says the Mac osx base system appears to be OK. However, when I run repair disk (on the external SSD) it says 'Error: partition map check failed because no slices were found'.

What can I do? Any help appreciated.

The Imac is late 2012....and can take up to OSX Catalina (which is what I was using)

Here are some photos of what's on the screen:

iMac 27″

Posted on Nov 13, 2024 10:10 AM

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Answer 1

HWTech

Level 9

60,464 points

Nov 14, 2024 11:01 AM in response to gkmach1ne

You are booted to a very old macOS installer so it cannot recognize the new drive layout & APFS file system used by macOS 10.13+. If the drive you are scanning is an APFS volume, then you should be booting to the most recent version of macOS compatible with the system which would be Catalina. Within Disk Utility you will need to click "View" and select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility. Run First Aid on the hidden Container for the drive you wish to check since it will also run automatically run First Aid on all APFS within that Container. You may also want to run First Aid on the physical drive as well.

Even is the First Aid summary says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are errors (or warnings), then run First Aid again until they are gone.

FYI, ignore all items below the line on the left pane of Disk Utility since they are only image volumes related to booting the macOS installer & are irrelevant.

A "slice" is generally only found on a drive within a RAID...is this OWC external drive in a RAID set? If so, then it seems like the other drive may have failed.

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Answer 2

HWTech

Level 9

60,464 points

Nov 17, 2024 1:51 PM in response to gkmach1ne

You can ignore those two warnings with the Container. I don't know why Apple even considers the first one a warning since that should be expected. The second warning is just informative because First Aid is not performing that particular check.

Make sure to try running First Aid on physical external drive (OWC Aura Ultra III Media item) to see if it will fix the partition issue while you are booted to another drive or Recovery Mode. Many repairs cannot be made while you are booted from the drive you are scanning.

If you want to always boot to the external drive, then make sure the external drive (or whatever volume you want as the default) is selected as the default Startup Disk System Settings. Keep in mind if you perform a PRAM Reset or clear the NVRAM, then the default Startup Disk will be reset to nothing which will cause the system to scan for bootable volumes and by default try to boot from the first internal bootable volume it finds (if none exist, it will choose an external boot volume if available).

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Answer 3

gkmach1ne Author

Level 1

39 points

Nov 17, 2024 5:51 AM in response to HWTech

Hi again,

Firstly here is the overall view from Disk Utility

These are the only two warnings (no errors) that I found.

I ran First Aid three times subsequently but the warnings remain.

When I ran First Aid on the whole volume I got this message:

But otherwise all seems outwardly OK....I'm logged in and the machine is on.

Other than running these first aid operations is there anything I should be doing to prevent this happening again?

Thanks

GK

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Answer 4

gkmach1ne Author

Level 1

39 points

Nov 14, 2024 1:16 PM in response to HWTech

Hey @HWTech

Thanks for the response.

I just came home and was going to try what you said and started the machine...and the apple logo appeared!!....and then in startup the disk showed up!! So I logged in while I could. It's weird. I've not tried any of your suggestions yet....I'm currently running a back up...because no way of kmnowing how long things will be OK for. Will try your suggestions from Disk Utility ASAP...will be interesting to see if any unfixed errors do appear. Thank you for your help...really appreciate it because it's an area I'm not 100% confident in.

Will report back

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Answer 5

gkmach1ne Author

Level 1

39 points

Nov 18, 2024 12:58 AM in response to HWTech

Thank you again, this is very helpful.

Good shout with the scan of the main external drive in Recovery. I will try this.

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