Is my hard drive missing if a iMac 2011 purchased at a garage sale boots up with a flashing grey question mark? I bought a iMac 2011 at a garage sale. But when I boot it up, I get a flashing grey question mark. Disk utility can't find Macintosh HD. Probably owner deleted files and HD? Do I need to install SSD drive to reinstall Sierra?
Try this: Press and hold Option+Command+R or Shift+Option+Command+R at startup and use Internet recovery mode.