There is a scenario where i didnt think i could encounter in my years of computing, especially on a newly purchased ridiculously expensive 2019 macbook pro. Not knowing how Apple's hardware (T2 chip) works made this a tiny bit challenging when working on this 2019 Macbook pro. There was a happy ending to all of this but it took almost eight hours in clicking and waiting on download of a new Mac OS, and finally reinstall the latest MAC OS Catalina which differed from the original factory de facto OS. This was a bit of a hack and a bit of luck in revitalizing a Mojave into Catalina OS over the course of roughly a day.
Assuming a Macbook pro hardware would function just like an PC, i made a boot USB Catalina earlier in order to prep for a new OS install. There were obviously knowledge i wasnt aware of, when doing this install. Assuming installing a clean version of mac OS X Catalina would be the same as installing a new ubuntu linux.
After holding on the "option" button to boot the USB, it prompted me with a message informing me that i could not boot USB due to some security item i must first resolve or disable. Not caring what that was, i then rebooted this macbook pro using "Command + R". I ended up with a similar screen and started to use the Disk util software to wipe the native SSD disk. After formatting this disk to APFS and renaming the disk to something a bit more obvious, i then exited out of disk util and returned to the main screen. At this point, i was ready to install Catalina onto this new disk. Without realizing that i had just wipe the factory recovery partition, along with the existing Mojave OS which i didnt cared about and had yet to allow to boot external USB and set security to medium. I panicked for around 5 mins due to not able to boot into this Catalina USB after several reboots !
After several reboots trying and hoping to boot external USB, all these reboot failed miserably. Each of the screens came immediately after each reboot went directly to a screen prompting me to enter a wifi name and a password. There was nothing i could do to inform this POST that there is a USB connected to a type-C port on the machine. At this point, scratching my head was the next best thing for me to do. Now, the damage to this laptop has been done but this wasnt completely lost. I wasnt quite sure what this wifi screen was about initially. The good thing was that apple provided this wifi screen was for a last resort to install an OS directly from Apple.com. As long as a mac owner can connect to a wifi network without username and password credential set, this was satisfactory on the reinstall side. The wifi screen required a wifi network name and a router password for it to connect to apple.com for it to download a new leaner Mojave OS.
After some network magic, i was able to download a version of Mojave OS over DSL, but this isnt what you would call a safe approach. i didnt know whether it would be suffice to download something this large (5.5 - 7.5 gigs) over a 400-500 KBps network. Also, i wasnt sure if there is an internal bittorrent kind of software when streaming this large OS file rather than a simple ftp transfer kind of thing from apple's end. This certainly was some apple magic, but i was glad this was provided from Apple since T2 chip prevented me from booting from any external USB !
After realizing that i could use my local network by entering the network name and its password. I then gained entry to a boot software after holding "Command + R". Thirty or so minutes went by, i could now see that boot menu screen started to appear. I then had chosen an option to reinstall Mojave. I clicked to reinstall the factory Mojave OS but this literally took a long time over DSL. Once the download was completed, all of the boot software began to conduct a reinstall. Much relieve at this point, since the OS installation process was working after a long awaited download from some random apple place. There was nothing to inform me of where this Mojave OS was coming from, by looking at the re-installation menu screen. One has to trust apple at this point, probably even much earlier.
ultimately, this reinstallation using apple's approach with their recovery download network worked !!! At this point, i still dont have Catalina but i do have a USB to boot Catalina. With much hope, i found that there was a menu item i would need to conduct first once i see a Apple logo after a reboot by using "Command + R". I held down "Command + R" then a boot software came up, i immediately looked for a menu item called "Security utility". After clicking this drop down menu, i was presented with another dialog box to disable FULL-SECURITY and enabled USB boot. Woot Woot, Success ! https://support.apple.com/en-us/HT208198#open
Finally, we are getting somewhere logically and there is light at the end of this tunnel. I rebooted once again, i held down the "option" button and MAGICALLY booted into this usb with Catalina. Now, i then deleted this disk and used this "disk util" to recreate this blank slate for Catalina with APFS. After more clicking and waiting on this clean install of catalina, macbook pro rebooted and came up with Catalina.
The lesson was not to wipe disk after holding down "Command +R" but to click on the "Security Util" on the menu item and enable the external boot along with set the security to Medium on all apple certified boot ISO downloaded from apple store. After a reboot, this second phrase, we can now hold down the "option" key to install a new OS using USB.
