Which drive to I select to completely erase my system's hard drive? Apple's instructions for completely erasing a system say to "select your startup disk". But my system shows a hardware drive icon and some gobbledygook as a name, and under that another icon and name that corresponds to the one I'm familiar with as my "disk". 



Which of these is the "startup disk" I'm meant to erase?



FWIW, when I attempt to erase the former, I get an error that it "can't be mounted"; but I can start the deletion process for the latter.
One of these icons represents the hard drive (the one with gobbledygook as a name), whereas the other represents a partition inside that hard drive. Usually there's a single partition per drive, but some power users may have reasons to create more than one. If you fall into the default case of a single partition, and you intend on staying that way -- or just want to erase your personal data and don't care how the drive will look like so long as your data's not in there -- then just go ahead and format the partition (the one for which you can start the deletion process). All of your data will be gone; maybe there's also a recovery partition hidden from view, which won't be affected by this process, but it won't contain any of your personal data.