I have been extremely low boot times on my 21.5in iMac, kind of a big case of buyers remorse, should've got internal SSD and more RAM for work but oh well. I currently have the 1tb standard HD and boot times are just so slow going back and forth from my MBP to this is hard. I have two questions. 1.) Is it possible to boot from an external HD on Yosemite? 2.) Is it worth it to invest in a 128/250gb External SSD to boot off of? I can't find the speed of my current hd anywhere and trying to test it results in errors, although I did find that my desktop replacement USB 3.0 HD is 180mb read & write.