I have a very weird problem – I messed up with the partions when I installed Bootcamp.
I want to upgrade my 10.7.5 to 10.8.5. I have bought the 10.8.5 from Apple Store. I downloaded it and when tried installing it, it failed because there was no Apple-formatted disk.
So I guess I have to reformat the whole drive. I have backed up all my data (I don't have enough hard drive space to make a Time Machine backup). I have just saved necessary files and also the installer for OS X 10.8.5 that I just downloaded from Apple.
Now, how can I reformat and install OS X from the installer placed in the external hard drive?
Best Answer
You don't need any external drive. You can erase the drive and reinstall OS X using the recovery system:
Some older Macs support a recovery partition but not Internet Recovery mode. See http://support.apple.com/kb/ht4904. If you have one of those Macs, you erase the whole drive from the recovery partition, and it is not in fact possible to reinstall OS X without starting up in Internet Recovery mode after that, you can probably use Recovery Disk Assistant instead.
If others search for how to do a normal erase and reinstall, erase the main OS X partition (like Macintosh HD) instead of the whole drive:
See https://support.apple.com/kb/ph11273.
If your Mac came with Lion, the Install OS X option will download and install the latest minor version of Lion. You can upgrade to Mountain Lion or Mavericks from App Store after that.