If you use Time Machine as your means of backing up your hard drive, don’t forget Time Machine is tough to test, and is very hard on hard drive directories. I recommend a “double back up” one Time Machine backup and one clone backup. (I recommend Super Duper) That way if you ever have to get right back to work (during a hard drive emergency), you can boot off your clone hard drive and find your current files from an hour ago on time machine. A full Time Machine restore can take 4 or 5 hours on a large volume AND it could run all day, and then not work (been there done that!). All of that time would then be wasted. If you had a clone AND another computer, you could boot off of your clone back up volume and get right back to work. See this MacMedics Blog post for more info.