Backing up to an internal drive of another machine
You have two options to allow Time Machine on your Mac to use an internal drive on another Mac Pro as the destination for normal backups from your MacBook.

Neither is a great option, but here they are.


Load OS X server on the Pro and set it up minimally to share that drive as a Time Machine backup destination. Your Mac will see the drive as long as the two computers have any sort of network connection and your Mac will quite happily send files to it's server as if it were a Time Capsule or other NAS device that supports Time Machine.
Shut down the Mac Pro and boot it in Target Disk Mode. This involves less expense (if you already have a FireWire or Thunderbolt cable) and less setup time (server could take hours to set up the first time) but it means you can't use the Mac Pro when the MacBook needs to back up. It also means you can't use the ethernet (or any other network) to perform the backup. Thunderbolt is practically limited to 2 meters by the longest copper cable being sold and the max length it could be is 3 meters until a fiber optic version of the protocol gets released. FireWire can work up to 100 meters, but cables that long are pricey and rare.


Since neither of these is a good option, you could also look at getting some free software like CrashPlan. This software would be installed on both computers and each could back up to the other (or you could only back the portable to the Pro - one way backup) and you could have all the files stored on the one drive.

The downside is it's not Time Machine - but it is quite solid software that many, many businesses depend upon for valuable data.

Sorry there isn't a quick - do this and that to make the Pro serve up a drive as a Time Machine destination - but hopefully one of the three options will let you work out a plan.