I bought an external hard drive to back up my MacBook files. I have set the energy tabs to never turn off, but no matter what I do, after about 1/2 hour the computer shuts down and the backup fails. In fact, the last backup I was able to complete was February, 2012. How do I stop the computer from shutting down before the backup is complete? evidently it wll take 10-14 hours to complete.