If you have erased or replaced your startup disk, you can use “OSX Recovery”
You might need to reinstall OS X for these reasons:
- You see a prohibitory symbol at startup.
- You inadvertently installed a version of OS X that’s older than what came with your Mac.
- You erased your startup disk, or are setting up a new startup disk.
Some tech pundits recommend a clean install of OS X when a new update has been released. Apple released OS X El Capitan on September 30, 2015. This can be a good opportunity to clean out the hard drive of accumulated and outdated system files in preparation for update, particularly with older computers.
Follow these links to Apple’s support pages:
If you need to reinstall OS X, you can use one of these methods:
The new version of Apple’s operating System “El Capitan” is now available for public install. Follow the link below for install instructions:
The following link to macworld’s pages provides a option to build a usb bootable version of the newly released OS X:
It is widely reported that the download of El Capitan is very slow. One option if you are planning on upgrading multiple macs is to use the usb bootable installer process, linked above, so that you only carry out one download and then locally install each computer.