Why isn't APFS partition on External drive not mounting?

I have an iMac with a damaged Fusion drive that was running Catalina, which I couldn't boot up because I got a Prohibition error message. Currently the iMac is booting from an External drive running El Capitan

Goal: make a clone / disk image of the Fusion drive to an External drive

I have a Macbook Pro running Catalina that I used to make an External bootable drive with two partitions

Partition 1: ( Slim 100 ) as a bootable drive running Catalina using APFS format
Paritition 2: ( Slim 900) for cloning contents of a damaged internal drive using HFS+ format

The Problem: On my Macbook Pro running Catalina, I can see both partitions ( Slim 100 | Slim 900 ) BUT on the El Capitan external drive used with the iMac only Slim 900 partition is mounting?

Is it due to one running El Capitan and one running Catalina or because I should have formatted drive differently?

Posted on Jan 12, 2021 11:35 AM

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3 replies

Barney-15E

Level 10

122,306 points

Jan 12, 2021 12:01 PM in response to Hanleymade

El Capitan has no idea what APFS is.

High Sierra and later are the only ones that can deal with APFS.

Is there an actual problem with the fusion drive, or do you just need to reinstall the OS. You should be able to do that from recovery

Hanleymade Author

Level 1

8 points

Jan 12, 2021 4:22 PM in response to Barney-15E

I got a Prohibition sign on the 3TB Fusion drive on the iMac running Catalina. When I rebooted into Recovery mode and to reinstall the OS it said Mountain Lion instead of Catalina so I stopped before the installation started. ( I assume that's because it originally ran Mountain Lion, because it's a late 2013 model iMac?

When I look at the Fusion Drive using Disk Utility, it shows a 128 SSD drive but not a 3TB Fusion drive.

I used disk drill and the Fusion drive is still there as "Macintosh" and "Macintosh-Data", but when I tried to reboot it it kept showing the Prohibition sign.

I thought I should clone the Fusion drive before trying to recover data OR should I just do the Reinstall OS X and then update from Mountain Lion up to ??

I've heard that Catalina has some issues with Fusion drives but Mojave works better with them?

Barney-15E

Level 10

122,306 points

Jan 12, 2021 5:08 PM in response to Hanleymade

If it shows the Apple 128GB SSD (or whatever) and not "Fusion Drive," I would imagine the HDD has died.

Whether you'll be able to recover the data is suspect, and reinstalling will likely only install on the SSD.

And, the fact that it wanted to install the OS that shipped on the Mac indicates the Recovery partition is invalid (likely died with the HDD).

Macintosh and Macintosh HD - Data does not indicate a "Fusion" drive. It is how the OS is split up on Catalina and Big Sur.

I can't tell you what disk drill shows you as I don't use it. However, being able to see anything might imply the HDD hasn't completely died.

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Why isn't APFS partition on External drive not mounting?