You should never edit crontab directly. Use:
to edit the crontab for the user currently logged in.
I can recommend to put your backup command in a script file, test it and only then place it in crontab. It is no fun to wait 12 hours and see the command failing.
Note that you cannot make any assumptions about paths and environment in cron. Use absolute path names for every command and file in your bash script.
jlinkels