I think my question was misinterpreted. That a backup can be done using a MAC/PC and iTunes, was clear for me. So the connection looks like iPad----MAC/PC----external HD
My preferable connection would look like this:
iPad----external HD is that possible?
ICloud sounds like a good option though.
But you can always backup and then copy that folder to the external hard drive just for safety. OR you can use iCloud as hazy suggested, I prefer iTunes because backups are stored locally and data can be extracted if anything