Do I need to unmount the internal HDD to prevent conflict?

Do I need to unmount the internal HDD to prevent conflict with the external SSD I use as start up disc? The iMac shuts down occasionally and this is the only thing I can think of which may cause this.

iMac 21.5″, macOS 14.5

Posted on Jul 24, 2024 2:53 PM

Similar questions

Restart after choosing an external hard drive as startup It seems that I haven't configured the external drive properly before choosing it as a startup. Therefore, upon restarting, the iMac showed a circle with electric flash roaming around the screen
352 3
Battle of the Disk Utilities (Multi-Boot APFS Internal SSD) Hello all, I recently upgraded an internal Fusion drive to a 2TB SSD in a 2017 iMac. The fusion drive contained three volumes/partitions: HFS+ (High Sierra/32-bit app support), Catalina (APFS) and Ventura (APFS sub-container). I used Carbon Copy Cloner for High Sierra and Catalina but decided to skip the Ventura APFS container because I intend to upgrade Catalina and no longer want a third macOS. First thing I notice after booting my upgraded iMac up, is that a 28GB partition showed up on the SSD. It appeared as a third choice when holding down "option" at startup, but upon selecting it the boot did not complete. Then, upon running Disk Utility, it didn't even *see* this partition. After a lot of hunting around to figure out how to "reveal" the partition using Terminal, I can now erase or re-partition the 28GB in Disk Utility — but am unable to delete it. I tried a force-removal in Terminal, multiple boots into Safe Mode, running Disk Utility in Recovery, reinstalling macOS on the bootable partitions — and still I can't get rid of this 28GB partition and return the free space to the remaining volumes. As part of the troubleshooting, I noted that despite booting successfully with no apparent issue into High Sierra and Catalina, Disk Utility throws all kinds of errors. It ranges from the apparently common issue of drives refusing to dismount so that First Aid can execute repairs to reporting that one of the volumes (Disk3s1) is corrupt. In booting from an external USB key containing various macOS installers, and launching Disk Utility from there, I was able to delete faulty Time Machine snapshots, and that got rid of the scariest message about my High Sierra volume being corrupt. (I was on the verge of wiping the drive because that sort of message from Disk Utility tends to mean there is no fix. Fortunately, there was.) Unfortunately, there's more: When I "view all" in Disk Utility (Ventura edition) to test the parent SSD while booted from a thumb drive, it reports errors on the parent volume but when I run other versions of Disk Utility, it does not. Conversely, when I run Ventura's version of Disk Utility on the individual APFS containers, there are no warnings/errors but when I use earlier versions of Disk Utility there are so many warnings on Disk3 (High Sierra container) that Disk Utility reports that it has suppressed the duplicating error entries. Depending on the version of Disk Utility, I get results that are all over the map, ranging from nothing found to orphaned files, warnings about invalid files (inode_val: object…), dstream, unable to verify the drive completely, references to suppressed warnings, etc. — only for Disk Utility to report that my volumes are "OK". I then decided to boot into Single User Mode, only to find that fsck_apfs -y can't be run in Single User Mode (unable to dismount the volume). However, I did find in Single User Mode a repeating entry called "sanity check" that reports an "fs_alloc_count mismatch" at the root nodes. (Caveat being, I can't be sure if that's a valid error, either, because it's not running off a fully dismounted SSD.) My last ditch effort was to search for an alternative to Disk Utility. I ended up buying an app called DriveDX. It reports that the drive health is good, with no errors of any kind. (Back in the day, I would have turned to the likes of TechTool Pro or DiskWarrior but apparently Apple never released the full APFS documentation so competitors to Disk Utility are unable to rebuild APFS volumes. Consequently, I am unaware of any third-party software that overcomes these limitations.) The original Fusion drive was itself new and had no Disk Utility errors before cloning. (I say "new" because this 2017 iMac was purchased off of an auction site still new in a sealed Apple box less than two months ago, probably one of the last 2017s on the planet with no mileage on it. My only reason for upgrading was to get a larger internal drive. Ugh.) In the end, I suspect this ties into the fact that I failed to exactly duplicate the original fusion drive. Although I can't be sure why I ended up with a persistent 28GB partition that I can't delete, I suspect the presence of allocation mismatches reported in Single User mode may explain some (if not all) of this. Q: Since the latest version of Disk Utility available to me (Ventura) doesn't find a problem with the bootable containers, should I worry about the fact that other versions of Disk Utility report numerous "warnings"?
4614 10
How do I erase an external HDD on my MacBook Pro? How do I erase an external HDD clean if I get this message on my 2021 MacBook Pro: "Disk Erase Failure, Couldn't unmount disk". [Re-Titled by Moderator]
389 5

3 replies

MartinR

Level 7

26,490 points

Jul 24, 2024 5:52 PM in response to K Walder

There is no conflict. The shutdowns are being caused by something else, not the fact that the internal HDD is mounted. When booted from an external drive, the internal drive would just be seen as an available data drive.

What external SSD are you using, what enclosure is it in, how is it connected to your iMac and is a USB hub in the chain?

It would also help to know the exact model iMac you have.

Jul 24, 2024 4:08 PM in response to K Walder

I don't believe that the internal drive should cause a conflict once the system has properly booted from an external source. The things that you might want to check is the connection to your external source (the communication cable and power source) and any logs in the console app that might display why these random errors are happening. You can use the disk utility to run a file system check by selecting the drive and using the first aid button to scan the drives internal and external and make any necessary basic repairs to the file system. The logs in the console should guide you in the right direction to find out what is happening. I hope that this helps you to resolve the problem.

rkaufmann87

Level 10

149,396 points

Jul 24, 2024 3:47 PM in response to K Walder

There is no context to questions so please look over Writing an effective Apple Support Communities Question and then re-write.

This thread has been closed by the system or the community team. You may vote for any posts you find helpful, or search the Community for additional answers.

Do I need to unmount the internal HDD to prevent conflict?