Unable to Install macOS on External Drive

macrlz9

macrumors 6502
Original poster
Dec 6, 2003
428
31
Long Island, NY
Hello!
I’m having a strange issue I’ve never had before. I have a 16” MacBook Pro and am trying to install macOS 10.15.3 on an external USB flash drive (connected via Apple’s USB-C to USB adapter.

  • I boot into Recovery (Command R)
  • Disable all T2 features (security and allow external drive booting)
  • Erase the drive GUID APFS
  • Select the USB drive to install macOS
  • It goes through the install but reboots and gets stuck at the Apple logo and progress bar (without time remaining indicating that the actual install has completed and is trying to boot). The machine will eventually reboot and get stuck again then reboot, etc.
I’ve also tried using Super Duper to clone an existing Catalina install to the USB drive which is also not quite working. It was doing the same boot loop but if I use safe mode (Shift), after an extremely long time... like an hour, the login screen will come up but it’s so unresponsive I can’t even login. After entering my password, it just eventually gives up, screen goes blank and machine either shuts off or completely locks up.

I’ve installed macOS on an external drive so many times and have never had a problem.
 

Fishrrman

macrumors Core
Feb 20, 2009
19,455
6,751
Sometimes... sometimes... it's "just that particular drive".
One drive won't work, while others will.

Have you tried any OTHER USB drives?
 

macrlz9

macrumors 6502
Original poster
Dec 6, 2003
428
31
Long Island, NY
Sometimes... sometimes... it's "just that particular drive".
One drive won't work, while others will.

Have you tried any OTHER USB drives?
This particular drive has lived as a usb macOS installer and works just fine that way so I didn’t really think of trying a different drive... I will give that a try and report back!
 

macrlz9

macrumors 6502
Original poster
Dec 6, 2003
428
31
Long Island, NY
Just realized I forgot to report back. Yes! I tried an old spinning USB2 hard drive and it worked just fine. Go figure! The flash drive I was trying to use works fine as a bootable macOS installer, just won’t boot if macOS is installed onto IT. Weird!
 

gilby101

macrumors 6502
Mar 17, 2010
316
125
Tasmania
Just realized I forgot to report back. Yes! I tried an old spinning USB2 hard drive and it worked just fine. Go figure! The flash drive I was trying to use works fine as a bootable macOS installer, just won’t boot if macOS is installed onto IT. Weird!
Power is my guess. The drive will be much more active as boot disk (compared with just an installer) and that consumes more power. And the use of a converter dongle doesn't help.
 
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