Reinstalling MacOS X

Locked out of an Apple iMac

Today I was called by a friend in Beck Row who’d been using his iPad for so long that he’d almost forgotten about his iMac which had been sitting in his office unused for several years. However, some things are best done on a large screen, and his iMac had a 21.5-inch screen, so he’d decided to start it up. Unfortunately his user account password didn’t work, even though he knew exactly what it was, so he asked if I could help him get back into the system.

Encrypted hard drive

After double-checking that we were typing in the user account password correctly, it became clear that the hard drive was encrypted with Apple FileVault. There was a hint for the encryption key, but it just said ‘a packet of crisps’. The hint meant nothing to my friend, as someone in the shop where he’d bought the system had set it up, so we were stuck. We tried a few wild guesses based on the hint, but none of them worked, so we had to find another way in.

Starting again from scratch

As there was no valuable data stored on the hard drive, we decided to wipe the whole thing and start all over again. However, this was not as simple as you might imagine. When you start up an iMac normally, or in ‘recovery mode’, it not only loads the MacOS software from the hard drive, but continues to access the hard drive while the software’s running. It’s impossible to wipe the hard drive while it’s in use, so we needed to start the MacOS software in a different way.

Loading MacOS from the Internet

Fortunately it’s possible to load MacOS from the Internet by turning on the iMac then quickly holding down the ‘Option’, ‘Command’, and ‘R’ keys before the iMac plays the startup chime sound. It takes longer to load MacOS over the Internet than it does to load it from the hard drive, but the hard drive is not then being used so you are free to wipe it. After wiping the hard drive and asking MacOS to reformat it without encryption, we downloaded and reinstalled MacOS.