As a huge e-hoarder that I am, I do happen to have multiple drives inside my computer. One of them locks up the POST process, forcing me to disconnect it and then hot plugging it later (figures!). Also, sometimes I do swap a drive for another and forget to reconnect the old one. As all my internal drivers are listed on fstab, the boot obviously locks if I do not use an specific flag: nofail.
There are other options, though. You may want to specify options for your device, but do not want it to automount at the boot, so you give noauto a try.