I had an SSD with mojave installed on it, and an iMac with Mojave as
well. And i tried booting from the SSD and updating to big sur, but
during the first restart (during the install) the computer just didn't
start up. And i tried booting an SSD with big sur installed on the same iMac and the same thing happened it won't boot and just shuts down, until i unplugged the external drive. Can anyone help me with this problem?
iMac, macOS 10.12
What exactly is this iMac model?
Try restarting from Recovery. See if there is "Startup Security" option there. It may or may not be there, depending on the mac model. If it is there, you will have to change the settings to allow booting from an external drive.
When connecting the disk to mac while it is turned on, i see the message "incompatible disk" "this disk uses features that are not supported on this version of macOS". I'm using macOS mojave 10.14.5. But shouldn't it still be possible to boot from it at startup? (before booting into mojave)
I already tried that, and there is only one boot option to disable boot from external drives (if i remember correctly).
My iMac model is: iMac (Retina 5K, 27-inch, 2019)
And booting from SSD with mojave worked just fine.
boot Big Sur from external SSD drive on iMac with Mojave
