Rsync with automatic stop if destination drive is missing

Filling the SD with what?

To me, rysnc should just stop or stall if is "sees" no external device, not "do something until full".

Does the other side have a rsync server running or are we going via ssh?

rsync -a --delete -e ssh remoteuser@remote.example.com:/this/folder /into/that/folder/

if I'm not mistaken in case there is no rsync server. Be careful though. rsync -n option allows a "dry run" to test everything.

Ah Ok so it works, but on the wrong folder.

Yes it's tricky:

Some examples:

rsync -a /source/from/folder /destination/to

...copies ONLY THE folder "folder" INTO the folder "to"

rsync -a /source/from/ /destination/to

...copies everything INSIDE "from" INTO the folder "to"

rsync -a /source/from/ /destination/to/

..SAME!

rsync -a /source/from /destination/to/

...copies ONLY THE folder "from" INTO the folder "to"

If you look at it, it's logical but we expect "those folders must be made identical".

Perhaps try this.

Create a script that checks for the existence of a zero byte, read only, and immutable file in the root of the destination drive. If the file exists, run the rsync command as usual. If the file does not exist (meaning the drive is not mounted) abort the job and email the server's owner.