I just got my first Macbook yesterday and after trying everything out, I began setting up Time Machine. I flipped the slid the switch to the on position and the pop up for selecting the back up disk appeared. But I couldn't find a disk to choose from but I know I obviously have a working hard drive. It's puzzling because I thought Time Machine should be a breeze to set up. I'm just a beginner so maybe it's a setting I overlooked. Could anyone give me hand with this?