You can make a difference in the Apple Support Community!

When you sign up with your Apple Account, you can provide valuable feedback to other community members by upvoting helpful replies and User Tips.

Looks like no one’s replied in a while. To start the conversation again, simply ask a new question.

User profile for user: NightGallery
NightGallery Author
User level: Level 1
8 points

External HDD Drive Mounts But Will Not Boot

Problem: iMac will not boot from external USB 3.0 drive. My main system is a 3.5 TB volume that is on the external drive (named “Crescendo"). The problem began when an unknown malfunction caused the Mac to restart. It then booted from the iMac's internal drive. On the iMac’s internal drive desktop screen, the external drive shows up on the desktop and in the startup disk window. However, there is no OS listed under Crescendo’s icon in the startup window in system preferences. Whenever I select the external drive as startup it glitches when attempting to boot and defaults to booting from the internal drive again.

Thinking that perhaps the external drive had failed, I bought a brand-new replacement and proceeded to restore from a Time Machine backup. This also failed at the last step and it booted up from the internal drive again. I then suspected maybe the drive enclosure was corrupting the operating system. So, I reformatted the disk, bought a replacement USB 3.0 enclosure and tried restoring in the same way again. Same result: The external drive shows up on desktop and startup disk window, but no OS is listed and the computer will not boot from the external drive. I have tried disk utility/first aid in Recovery Mode. I have also tried reinstalling OS in Recovery Mode. No matter what I’ve tried the computer will not boot from

the external drive. I zapped my NVRAM, too. None of these have solved the problem. Any help is greatly appreciated. Please see attached screenshot of Startup Disk window in System Preferences.

iMac 21.5″, macOS 10.15

Posted on Apr 16, 2021 8:45 AM

Reply
Question marked as Top-ranking reply
User profile for user: den.thed
den.thed
User level: Level 10
82,570 points

Posted on Apr 16, 2021 12:51 PM

You need to start over.


Option 1

  1. Enter macOS Recovery Mode.
  2. Open Disk Utility, click "All Devices" in the small "View" box.
  3. Select the External Drive and Re-format it GUID Partition Map / APFS.
  4. Quit Disk Utility and go to Install macOS Installer Catalina and Install macOS.
  5. Now startup from the new macOS on the external drive and go thru the initial set.
  6. Then Migrate from your Internal Macintosh HD macOS or Time Machine during the initial setup.

(please note that migrating from your internal drive is faster than miograting from TM on an external drive)


Option 2

  1. Open Disk Utility, click "All Devices" in the small "View" box.
  2. Select the External Drive and then Re-format it GUID Partition Map / APFS.
  3. Quit Disk Utility and use CarbonCopyCloner to make a bootable clone on the external drive.
  4. Now go to System Preferences > Startup Disk > select the clone as the startup disk and then click Restart.
View in context

Similar questions

5 replies
Question marked as Top-ranking reply
User profile for user: den.thed
den.thed
User level: Level 10
82,570 points

Apr 16, 2021 12:51 PM in response to NightGallery

You need to start over.


Option 1

  1. Enter macOS Recovery Mode.
  2. Open Disk Utility, click "All Devices" in the small "View" box.
  3. Select the External Drive and Re-format it GUID Partition Map / APFS.
  4. Quit Disk Utility and go to Install macOS Installer Catalina and Install macOS.
  5. Now startup from the new macOS on the external drive and go thru the initial set.
  6. Then Migrate from your Internal Macintosh HD macOS or Time Machine during the initial setup.

(please note that migrating from your internal drive is faster than miograting from TM on an external drive)


Option 2

  1. Open Disk Utility, click "All Devices" in the small "View" box.
  2. Select the External Drive and then Re-format it GUID Partition Map / APFS.
  3. Quit Disk Utility and use CarbonCopyCloner to make a bootable clone on the external drive.
  4. Now go to System Preferences > Startup Disk > select the clone as the startup disk and then click Restart.
Reply
User profile for user: den.thed
den.thed
User level: Level 10
82,570 points

Apr 16, 2021 11:02 AM in response to NightGallery

What steps did you follow to create this bootable startup disk?


I'm asking, because your external disk does not show a MacOS version in the Startup Disk preference panel.



FWIW my Crucial 1T SSD is currently in a Sabrent USB 3.0 enclosure, cloned using CarbonCopyCloner and it is bootable from both Alt (option) and from the Startup Disk preference panel. I also have two other clones on different external drives made with CCC and they are also bootable.

https://bombich.com


Also make sure if you have not already done so, is to allow external booting in the Startup Security Utility.

https://support.apple.com/en-us/HT208198

Reply
User profile for user: NightGallery
NightGallery Author
User level: Level 1
8 points

Apr 16, 2021 12:03 PM in response to den.thed

Steps taken to create new disk:

  1. Initialized HDD
  2. Formatted MAC OS Journaled with GUID.
  3. Entered Mac Recovery Mode
  4. Attempted to restore from Time Machine backup.
  5. All 3.5 TB Transfers and renames disk.
  6. Instead of booting from external drive when restarting, defaults to internal drive instead.
  7. Mounts and appears on desktop of internal drive system, BUT does not show an OS type or version below icon in Startup Disk window and will not boot from it.
  8. Attempt to reinstall OS Catalina on "restored" external drive.
  9. Again, defaults to internal drive upon booting after installing.
  10. After rebooting internal drive, startup disk does not show an OS type or version below icon in Startup Disk window and will not boot from it.



From all the research I've done, it looks like I formatted correctly and went through the proper steps to restore data from a TM backup and to reinstall an OS. My machine doesn't have the T2 chip so, the security utility you mention is not an available feature in Recovery mode for me. Thanks for your help!

Reply
User profile for user: NightGallery
NightGallery Author
User level: Level 1
8 points

Apr 16, 2021 4:52 PM in response to den.thed

Performed Option 1 to the letter and renamed drive "Zapple" Here's what happened:


It defaulted to internal drive upon booting after installing.

After rebooting to internal drive, startup disk does not show "Zapple" at all. See screenshots. The drive has install data, but doesn't complete a critical final steps that allows it to be bootable? I'll try option 2 now. Fingers crossed.



Reply
User profile for user: NightGallery
NightGallery Author
User level: Level 1
8 points

Apr 17, 2021 10:56 AM in response to den.thed

I reinstalled Catalina on the internal drive to eliminate any question of a buggy operating system being the culprit. Then, I performed Option 2 exactly as you articulated, because AI only have a TM backup of the system in question, I cloned the internal drive. Same result—mounts and appears on desktop of internal drive system, BUT does not show an OS type or version below icon in Startup Disk window and will not boot from it.. Strange, though; screenshots show cloned system data AND acknowledges presence of 10.15.7. I'm beginning to think there is something wrong with the iMac itself, hardware-wise. It is the 21" late 2013 model. I may try a brand-new disk one more time as a last ditch and then look for a proper repair shop. Thanks again for all of your help in troubleshooting this very mysterious issue.

Reply

External HDD Drive Mounts But Will Not Boot

Welcome to Apple Support Community
A forum where Apple customers help each other with their products. Get started with your Apple Account.
Learn more Sign up