iMac won’t boot from external SSD if other external drives are attched

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Level 1

4 points

iMac won’t boot from external SSD if other external drives are attched

I have three mechanical drives (one powered WD and two WD passport drives) attached to my 2015 5k Retina iMac. I boot to the Mac OS ( Big Sur) from a Samsung T5 1TB and it runs like a dream. My issue is I can’t boot with the other drives attached. I get the grey screen and the folder icon. Unplug the externals and it boots fine. Somehow the external drives are stopping the boot. Ive tried the whole unplug and PRAM zapping thing but it doesn’t work. Any help appreciated.

Posted on Oct 2, 2021 09:56 AM

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Answer 1

Oct 2, 2021 10:25 AM in response to Animator811

Are any of the drives powered by an external ac adapter, or are they self-powered and drawing power from the Mac through the USB(?) port?

It's likely that the combination of drives is trying to draw more power than the iMac can provide.

Consider looking into purchasing a powered hub to maintain the connection of all these drives to your Mac.

OWC/Macsales has a number of solutions for this at: https://eshop.macsales.com/shop/docks

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Answer 2

Animator811 Author

Level 1

4 points

Oct 3, 2021 03:45 AM in response to Animator811

Solved it! I reformatted all the mechanical drives to Mac OS Extended (Journaled) with a GUID partition. Boots straight to the SSD now.

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Answer 3

Animator811 Author

Level 1

4 points

Oct 2, 2021 10:28 AM in response to D.I. Johnson

One is a powered drive the other two draw power. I have tried all attached to the iMac direct and it still doesn’t boot.

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Answer 4

Jack-19

Level 8

46,176 points

Oct 2, 2021 10:53 AM in response to Animator811

Just to confirm; the Mac won't boot if the Western Digital drives are connected, correct?

Does it boot if only the Samsung SSD is connected?

Jack

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Answer 5

Animator811 Author

Level 1

4 points

Oct 2, 2021 11:07 AM in response to Jack-19

Yeah, if I unplug all the external drives apart from the SSD (This has the OS on it) it boots fine.

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iMac won’t boot from external SSD if other external drives are attched

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