My mbp is around 4 years old and getting sluggish. I am researching doing a clean install of Yosemite (i.e., backing up all data and formatting the hard drive and the reinstalling Yosemite). There is a lot of info out there on how to do this. My question is this: some guidance has me creating a "bootable installer drive". Other guidance doesn't mention that. Is creating the bootable drive really necessary? Thanks for your help.