Erasing and reinstalling boot drive on a Mac question

[tsinsf]

I want/need to erase my 2013 Mac Pro boot drive. I have it cloned onto an external hard drive and also backed up in Time Machine. I want to erase it and reinstall my existing OS, which is 10.14.6 (Mojave), NOT Catalina. Is this possible? Is the best way to erase it to usemacOS Recovery? If so, does it reinstall the existing OS that was on your hard drive before you erased it, or will it install Catalina? Thanks.

[Joe Towner]

Grab DiskMaker X https://diskmakerx.com/ and use it to create a bootable USB drive (sd cards work too).  To download Mojave click the link on this page https://krypted.com/mac-os-x/download-older-versions-of-macos-and-mac-os-x/ - it'll grab 10.14.6 as a full offline installer.  Hold down the Option key during boot and select the USB/SD card to boot off it. Then you can use Disk Utility to erase your internal boot drive - format it as APFS to make life easier later. Once the format is done, install MacOS Mojave. Then you can either migrate your user accounts back (from cloned copy of the drive, or the Time Machine backup) or do a selective copy/restore later.

[BobShaw]

Boot up holding CMD-R and restore from a Time Machine backup from Mojave should work.

[plugsnpixels]

Thanks for the handy link Joe. I was wondering if Apple keeps these installers updated in terms of the certificate, as sometimes our already archived copies of these OS's no longer launch due to expiration.

At any rate, I am downloading a few to replace my backups.

PS: I downloaded High Sierra and it was the same build as my archived version. I already know my Mojave installer is the more recent one so I'm good there.

[digitaldog]

You should be able to boot in Recovery mode (Command R when booting) and use Disk Utility to copy the clone or, you can use Time Machine, either can/should work.

[Joe Towner]

Thanks for the handy link Joe. I was wondering if Apple keeps these installers updated in terms of the certificate, as sometimes our already archived copies of these OS's no longer launch due to expiration.

At any rate, I am downloading a few to replace my backups.

PS: I downloaded High Sierra and it was the same build as my archived version. I already know my Mojave installer is the more recent one so I'm good there.

It's actually easy to change the date in the boot launcher before kicking off the install.  And Apple must have heard from people because they re-released installers for 10.10 onward good until 2029 - https://tidbits.com/2019/10/28/redownload-archived-macos-installers-to-address-expired-certificates/

[plugsnpixels]

Thanks Joe for the additional info!