Can I copy the mac OS directly from my macbook pro to a card in the SD slot?
You can use Disk Utility to make bootable copies of the OS. It's always preferable to use an OS that's not booted (i.e. you're not copying yourself), but in practice, OS X generally behaves well. Worst thing - you waste time with a non-bootable copy.

You could also look into something like SuperDuper or CarbonCopy Cloner. They will serve you for more than the initial need of making a backup copy of the OS.

Worst case, you end up with a broken system and have to pick up a secondhand disk from craig's list or similar. Apple also sells replacement media, but used media is usually less expensive.