The original SSD in the Mac was faulty and needed to be replaced. To do this, the user formatted and connected a USB external disc drive. They then used recovery mode to install MacOS Mojave on the USB drive and restarted the Mac with the USB drive as the boot drive. Next, they updated MacOS to Mojave, which fixed the firmware issue with the SSD. After shutting down the Mac, they removed the faulty SSD and installed the new Sabrent SSD. They then restarted the Mac from the USB drive with Mojave and used disk utility to partition the SSD. The user then downloaded MacOS Big Sur from the app store and installed it again, choosing the Sabrent SSD as the destination. After the installation was complete, they restarted the Mac and shut it down, unplugging the USB drive.