If you're using ZFS, the right thing to do is to attach drives C and D temporarily, create a second zpool on them, them use zfs send/receive to replicate snapshots from the primary drives to the backups. You can then export the zpool and move it to a different location.

Refreshing the backups is either done by putting the backups online at the remote location and syncing the deltas between the last snapshot and the current over the net, or by bringing the drives back to the primary, and sending the deltas.

The sanoid/syncoid toolset will help immensely with handing the necessary zfs snapshot and send/receive commands: https://github.com/jimsalterjrs/sanoid