Have you guys actually done this process? I have and it didn’t work. Took it down to my local Genius Bar and neither of the two “Apple Geniuses” that worked on it could get it to boot the external drive … and it wasn’t one of those flakey WD drives, either. The best that they could do was send in a report to Apple Engineering.
In fact, here is a copy of the report:
Genius Bar Work Authorization
Repair No: RXXXXXXXX
Customer Information ### Product Information
Terry Sneller Warranty Status: In Warranty (W)
Canada Model: MacBook Air (Retina, 13-inch, 2019)
Date of Purchase: 10-Jul-2019
Serial No: XXXXXXXXXXX
Problem Description/Diagnosis
Issue: Customer came to Genius Bar with a new MacBook Air and two external hard drives. Customer had used carbon copy to copy the internal hard drive of an older Mac to an external hard drive and then to a second external hard drive. Customer wanted to use the external hard drives as a bootable backup for his clients computers if they were infected with ransomware. Attempted to boot to external drives using his new MacBook Air, which has a T2 chip. We removed secure boot and allowed booting from external sources. After the setting was changed, the copied drives appeared in startup manager, but booted to a prohibitory sign. Tested external drive with our floater MacBook Air, not a T2 system, and it booted normally.
Proposed Resolution: Was unable to find any Apple articles referencing creating external bootable Mojave disk or how to make it work with T2 system, only an external installer. Recommended to customer to have his customers use time machine as it is is supported for all systems to back up user data. Customer pushed back as his customers leave their external hard drives connected and it would back up ransomeware as well if it happened. Advised customer to advise his customers that it may be better to connect hard drives only when they intend to create a backup to avoid that possibility. Also advised customer to contact carbon copy to see if they have any options as their software is what created the bootable disks that were not working for the customer.
Proposed Resolution: Application used by customer to create clones has a support article referencing prohibitory sign on startup.