For iSCSI see , for DRBD see .
iSCSI Target is basically a server that resembles a SAN storage and exports it's block devices via ethernet. Also rather cheap hardware solutions are available. The iSCSI Target should not be part of the cluster and it is a bottleneck as well as an single point of failure. iSCSI is more scaleable than DRBD.
On the cluster nodes you will need to configure a iSCSI Initiator, which is a client to read the iSCSI block devices.
DRBD basically is a RAID 1 over the net. Same downsides as iSCSI.
Both technologies make it possible that you have a new shared SCSI device available which you may format with a cluster filesystem.