I am planning to sell my 2011 11" mba soon to upgrade to the 12" rMB. I bought WD 500gb hard drive back in 2011 to help with back ups incase something went wrong. I admit I do not fully understand how it works and need some help. I was planning on wiping the external hard drive and starting over. I was thinking just using the extenral hard drive as one big drag and drop "back up" like its a usb stick over having time machine backing up everything. That way, when I do upgrade, I have my few important files (photos, music, word docs) that I can just drag and drop back into the new rMB. Maybe I don't fully understand how external HD's work, but what do you all think I should do?