More recent versions of the netcat command (nc) will not allow the -p and -l options to be used at the same time, so instead of:

nc -l -p 1234 | dd of=/dev/sda

 you would type:

nc -l 1234 | dd of=/dev/sda

If you are using nc with dd to transfer an image of a partition from one machine to the other, one of the problems is that dd and netcat won't show you a progress bar of the operation.  One solution to this is to install pipe viewer by Andrew Wood.  It then allows you to pipe the netcat command to the pipe viewer, allowing you to view the progress of the entire operation and for debugging.

target machine:

nc -l 1234 | pv | dd of=/dev/sda

source machine:

dd if=/dev/sda | nc 192.168.0.12 1234