In other thread on LQ I found that KNotes stores it's data in VCalendar format(textfile) in ~/.kde/share/apps/knotes/.
My idea is to copy this over a remote device, and make a symlik to it in your knotes directory.
The problem is, that if you remove or unmount the remote device, you wont be able to read your knotes.
I'd prefer some kind synchronization of this directory in example with rsync. This way you wont loose your notes if the device gets broken. Try
.