What is a back up disc?

Depending on the frame of reference, the backup disk/disc is usually an external

USB drive; where your Mac can use Time Machine; to create workable backup on.

• Back up your Mac with Time Machine - Apple Support

https://support.apple.com/en-us/HT201250

Related Article

• If you can't back up or restore your Mac using Time Machine - Apple Support

https://support.apple.com/en-us/HT204157

• Backup disks you can use with Time Machine - Apple Support

https://support.apple.com/en-us/HT202784

Learn about Other ways to back up and restore files.

https://support.apple.com/en-us/HT209174

Good luck & happy trails! ☺︎

Do I have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters.

A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS.

For addition purposes - two Tested Bootable Clones each to separate external drives.

This is insurance incase.

At least one Tested Clone and / or Time Machine Backup should be Off - Site