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Booting iMac from USB

Is it possible to boot an iMac from a USB the way you boot a Unix system?

I tried but it insists in _installing_ the system, not booting. I am trying to "see" the hard disk in a system that won't boot by itself, but I am trying to avoid reinstalling to make sure everything is as "untouched" as possible.

Thanks in advance.


(8 year old Intel iMac, I already have the startup USB ready)

iMac 27″

Posted on Apr 26, 2023 5:09 AM

Reply
Question marked as Top-ranking reply

Posted on Apr 26, 2023 7:38 AM

Alberto_FSoto wrote:

Is it possible to boot an iMac from a USB the way you boot a Unix system?


Yes it is possible to startup from a compatible macOS version on an external drive.

see > Change your Mac startup disk - Apple Support


What external USB drive do you have?

Does the iMac startup in macOS Recovery?


I tried but it insists in _installing_ the system, not booting. I am trying to "see" the hard disk in a system that won't boot by itself,


What happens when you startup the iMac?

see > If your Mac doesn't start up all the way - Apple Support


Have you tried Safe Mode to get the iMac working again?

see > How to use safe mode on your Mac - Apple Support


but I am trying to avoid reinstalling to make sure everything is as "untouched" as possible.


Have you tried reinstalling the macOS over the current version using macOS Recovery?

see > How to reinstall macOS - Apple Support


Reinstalling macOS over the top, does not wipe the user data like a full erase and reinstall. Besides you have a Time Machine or other backup to fall back on right...?

9 replies
Question marked as Top-ranking reply

Apr 26, 2023 7:38 AM in response to Alberto_FSoto

Alberto_FSoto wrote:

Is it possible to boot an iMac from a USB the way you boot a Unix system?


Yes it is possible to startup from a compatible macOS version on an external drive.

see > Change your Mac startup disk - Apple Support


What external USB drive do you have?

Does the iMac startup in macOS Recovery?


I tried but it insists in _installing_ the system, not booting. I am trying to "see" the hard disk in a system that won't boot by itself,


What happens when you startup the iMac?

see > If your Mac doesn't start up all the way - Apple Support


Have you tried Safe Mode to get the iMac working again?

see > How to use safe mode on your Mac - Apple Support


but I am trying to avoid reinstalling to make sure everything is as "untouched" as possible.


Have you tried reinstalling the macOS over the current version using macOS Recovery?

see > How to reinstall macOS - Apple Support


Reinstalling macOS over the top, does not wipe the user data like a full erase and reinstall. Besides you have a Time Machine or other backup to fall back on right...?

Apr 27, 2023 1:44 AM in response to den.thed

Thanks a lot for your answer and all the tips.


Trying to answer your questions. My iMac died over the weekend. I found a black screen with the "no parking" symbol on it, apparently some OS trouble. I have a current Time Machine backup, BUT there were some files I was working on (Office Word) that were not saved for a while (my bad, I know!). As far as I can see Time Machine keeps the last _saved_ version, but it does not keep the cached security versions that Word could use for auto-recovery. That is why I am trying to avoid reinstalling or rebuilding from the backup, as I am attempting to keep the hard drive as "untouched" as possible.


The iMac has a "Fusion Drive" made of a small SSD drive and a large HD. This could be part of the problem (more below).


Oh, and in case I need to specify: I am an absolute ignorant on this, I´m just trying to find all the info I can gather and play it safe. So I may be doing or assuming something obviously wrong, appologies if that´s the case.


  • I tried safe mode but it did not help, went to the same dark-no-parking screen
  • I have a 250Gb SSD USB for startup where I installed what I thought was a bootable OS, but turned out to be a installer--so it did not work for me (I´ll try the startup you suggested)
  • The iMac starts on Recovery. It shows only the SSD drive. I don´t know whether it means that the HD is dead/unavailable or that the "Fusion drive", whichever way it works, has somehow disassembled itself and cannot see the second part.
  • In fact when starting Disk Utility in Recovery mode I get a message like "Fusion Drive is down, do you want to restart it?" but it scares me to death that it may go and reformat everything (which is what I am trying to avoid) so I answer "No" every time
  • I can also start from an external drive. With the help of a colleague we tried several options and in the end we managed to boot from an Ubuntu USB. Again, it did not see the hard drive at all.


Reinstalling macOS looks like the way to go, but I did not feel safe that it does indeed ignore the user data completely. If I could open it I would just take the HD out and reinstall everything, unfortunately that is not a practical option!


Once again thanks for your kind help. I´ll check the links you sent


Cheers,

A

Apr 27, 2023 8:09 AM in response to Alberto_FSoto

You're welcome.


If you can not see the HDD in macOS Recovery, then it has failed and anything that was not previously backed up, is going to be very difficult and expensive to retrieve.


Difficult and expensive, as in having the HDD removed and sent out to a professional data recovery service.


My advise at this point, is to take the iMac into you local Apple Store or AASP and have it evaluated. Then decide on a repair or replacement and restore from the most current backup that you have.

Booting iMac from USB

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