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Old 03-20-2020, 12:33 AM
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JFreak JFreak is offline
Join Date: Jan 2003
Location: Tampere, Finland
Posts: 22,442
Default Re: Rolling Back O.S.

Yeah. I was a little interested about this so did some research. Have no T2 hardware here though, so cannot test.

Here's what you need to do, after you have created that bootable USB installer.
  • boot into recovery mode (command+R while booting)
  • in the menu bar, click utilities > startup security utility
  • select admin account and enter password
  • under "external boot" select "allow booting from external media"
  • close the utility and restart the computer holding option key
  • select your USB installer thumb drive
  • when it boots it'll say "software update is required to use this startup disk"
  • make sure you're connected to wifi, then select download. let it download. it will automatically reboot
  • on the second reboot, hold down option and select the USB installer thumb drive again
  • it will show the apple logo couple of times before the installer starts

This is how you should get your USB installer working with T2
What we do in life, echoes in eternity.
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Old 03-24-2020, 01:22 PM
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DetroitT DetroitT is offline
Join Date: Dec 2000
Location: Detroit, Mi, USA
Posts: 2,434
Default Re: Rolling Back O.S.

This is also needed for making external bootable clones.
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Old 04-01-2020, 08:21 AM
jaglance jaglance is offline
Join Date: Nov 1999
Location: acton ca usa
Posts: 27
Default Re: Rolling Back O.S. FWIW

Just rolled back a 2019 iMac to Mojave. No T2 chip. Had to use a usb 2.0 drive. For some reason boot drive with a 3.0 was undetectable
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Old 04-01-2020, 12:19 PM
Sardi Sardi is online now
Join Date: Aug 2002
Location: Perth
Posts: 2,458
Default Re: Rolling Back O.S. FWIW

Originally Posted by jaglance View Post
Just rolled back a 2019 iMac to Mojave. No T2 chip. Had to use a usb 2.0 drive. For some reason boot drive with a 3.0 was undetectable

Yes. I’ve had this problem before too. Couldn’t for the life of me work out why.

Turns out, it’s dependant on the type of USB3 enclosure you’re using. If you do a web search you’ll find a bit of info pertaining to the issue.
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Old 04-03-2020, 03:13 AM
markanthonybr markanthonybr is offline
Join Date: Dec 2013
Location: London, England
Posts: 11
Default Re: Rolling Back O.S.

I manage over 100 Macs for a University Music school here in London and just want to point out somethings from my latest experience:
1- the utilities security enable external boot only applied to Macs on sierra/ high sierra this wasn’t a necessary step with clean installs of Mojave it was an added security measure from apple in sierra/ high sierra.
2- If your mac is shipped with Catalina you wont be able to rollback to Mojave. This is based on my just having imaged 36 new mac minis in 2 of our mac labs.
3- target disk mode wont work using an original apple usbc cables only 3rd party cables still work. So carbon copy cloner running from an external drive worked fine for me.
4- Catalina will not make an older mac faster. Apples design model is to force you into buying new machines so I highly doubt the person who posted here that his iMac seemed faster because of the os update to Catalina is correct. He probably had improved performance from doing a clean install as wiping the disk and reinstalling will improve performance as bugs and permissions errors are put right or his mac was never configured properly. So I wouldn’t recommend that older mac users to rush into an OS upgrade based on that info.

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