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I've been running Mac OS Mojave (iMac late 2013) on an external SSD connected via USB 3.

Since I've had issues with the internal HDD randomly unmounting and also Mac OS telling me that RAM is insufficient while performing normal tasks (16GB installed, I assume an issue with compressed memory) I decided to completely format both disks and reinstall an older version of Mac OS with HFS+ file system (High Sierra).

Sadly, this did not help and I still have the unmount issue. Also the boot is still very slow (about 2 minutes). The HDD is completely empty with no files. I haven't really found any helpful tips on the Internet.

Thank you!

/dev/disk1 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS Storejet Transcend M... 499.2 GB   disk1s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s3

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS HDD                     999.9 GB   disk0s2
Julius2020
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  • There are several aspects to this question, so let's try to untangle them a bit first. I assume you reinstalled macOS on the SSD, can you add the output of diskutil list? The 2 minutes is which time, from power on til the initial login screen, from power on til Finder, or something else? The HDD and/or the RAM might just be failing, did you run any hardware tests on it? – nohillside Nov 09 '20 at 12:12
  • Hello, thank you for your reply. The 2 minutes is from Power on to login screen. I've not run the Apple Hardware Test yet, but I did run the first aid with disk utility both on HDD and SSD and it did not find anything to repair. – Julius2020 Nov 09 '20 at 12:32
  • Please edit the question to add additional information :-) – nohillside Nov 09 '20 at 12:33
  • Okay, yes I did. – Julius2020 Nov 09 '20 at 13:11
  • This shows the SSD, right? Not the HDD having issues (which probably is disk0). – nohillside Nov 09 '20 at 13:50
  • I remember seeing a very similar question a few months back. Can you please post the specific model iMac and it's config (27" or 23"), the storage is important like was it a 1TB fusion or the 512GB SSD. Please post the details and don't leave things out like disk0. Showing us info on what's not the issue doesn't help us. – Allan Nov 09 '20 at 14:42
  • Hello, thanks for your replies. I've added the info for the internal HDD. My system is a 27 inch iMac from late 2013 with the internal 1TB HDD, no fusion drive. I upgraded the pre-installed 8GB of ram to 16GB a couple of years ago. I've done an Apple Hardware Test, but it does not find any issues. – Julius2020 Nov 09 '20 at 17:32
  • See this answer. The problem that you have is the internal drive is failing, but it hasn’t failed completely. You can’t disable it meaning macOS is going to see it both mount and dismount. Booting from an external drive is a temporary fix, but what you have to do is go in and replace the drive. 1TB SSDs are super cheap now at around $100USD. Fix that and your problems will go away. – Allan Nov 09 '20 at 18:56
  • @Allan Thank you for your solution. Just out of curiosity, how do you know from the diskutil list it that the HDD is failing? Or is it based on your experience and the similarity to the case you've linked? I always try to expand my knowledge. Thanks. – Julius2020 Nov 09 '20 at 19:14
  • diskutil doesn't tell me that. It only gives me a picture of what you're working with. The driving failing is based on decades of experience with thousands of devices. In your case - the internal HDD should never unmount under normal boot. Intermittent issues, especially when mechanical HDDs are involved, are are always a failing drive. Never a controller (logic/main/motherboard) and the SATA cables in iMacs are "proper" cables, not super thin ribbon cables like in laptops. – Allan Nov 09 '20 at 20:15

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