To actually answer your question, as long as you just backup on the Mac, you will not lose any data. But turn off auto sync before you backup. On the Mac, launch iTunes and go to iTunes>Preferences>Devices>Prevent iPods, iPhones and iPads from automatically syncing.
In any event, you should get a message from iTunes that your iPad has been synced with another computer at which time you should be able to cancel anything that you are doing if necessary,
You might want to consider moving your iTunes library to the Mac and making that computer your syncing computer.
How to move your iTunes library to a new computer - Apple Support