Preflight error 21 during macOS recovery on external drive

Question

Lizbethan Author

Level 1

8 points

Preflight error 21 during macOS recovery on external drive

computer crashed

tried everything

recovery won't install

bootable start up and install flash won't install

my password is rejected

my network password is rejected wifi is connecting because here I am

disk utility says drive is fine

I have been using a bootable Samsung T7 external running OS Monterey for a year no problems (apple rebuilt internal fusion drive on 2017 intel imac aged quickly and failed - destroyed most of my data)

I've had no prior problem indicating my external working drive was having problems

[Re-Titled by Moderator]

Original Title: preflight error 21

iMac 21.5″, macOS 12.7

Posted on Oct 31, 2025 9:21 PM

Reply

Answer 1

den.thed

Level 10

90,370 points

Nov 1, 2025 8:21 AM in response to Lizbethan

Sounds like part of the old Fusion Drive has completely failed and/or now the T7 has a serious problem.

com.apple.buildinfo.preflight.error error… - Apple Community

https://iboysoft.com/howto/com-apple-buildinfo-preflight-error.html

If an error occurred while updating or installing macOS - Apple Support

If you can not get into internet recovery mode over the Wi-Fi connection..?

Connect the iMac directly to your Router using an Ethernet cable and try again.

Other options for moving forward include:

1) Take the obsolete 2017 iMac to an AASP and spend more than it is worth for a proper repair.

2) Replace the obsolete 2017 iMac with a new 24" iMac and then migrate into it using your backup.

Reply