I have a hdd connected to one of my raspberry pi’s that is on raspbian, now I would like to be able to backup my Linux machines 2 or more to that hdd on a regular basis to help with system restores. Rsync would probably be a good start from my searching but I need a hand with the proper syntax and switches .
I use the code in Backup image of SD Card to perform incremental backups.
You can backup to a network drive or local HDD.
There are a number of other examples using
can-a-raspberry-pi-be-used-to-create-a-backup-of-itself creates full backup images to attached HDD.
(Some of the other Answers in Can a Raspberry Pi be used to create a backup of itself? need to be used with caution)