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Helpful answers
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Apr 27, 2016 8:58 AM in response to rodion15by alex_h1,Hello rodion15,
Thanks for using Apple Support Communities.
If you want to partition your disk, you can do so while booted to it and use Disk Utility. If you want to reformat the disk you boot to, you will need to boot to recovery mode and reformat from Disk Utility.
Disk Utility (El Capitan): Partition a physical disk
Erase your drive and install OS X
Generally you don't need to erase your startup disk to reinstall OS X. The OS X Installer is designed to allow you to perform an install in place over the same version or earlier versions of OS X. If you're transferring your Mac to a new owner, you might want to erase your built-in startup disk before reinstalling OS X.
Important: Before you erase and reinstall, back up your important files. If you’re using a MacBook, MacBook Air, or MacBook Pro, make sure the power adapter is connected and plugged in while performing these steps.
You can use these steps to erase the files on your startup disk and install a new copy of OS X:
- Before you begin, make sure your Mac is connected to the Internet.
- Restart your Mac. Immediately hold down the Command (⌘) and R keys after you hear the startup sound to start up in OS X Recovery.
- When the Recovery window appears, select Disk Utility then click Continue.
- Select the indented volume name of your startup disk from the left side of the Disk Utility window, then click the Erase tab.
- If you want to securely erase the drive, click Security Options. Select an erase method, then click OK.
- From the Format pop-up menu, select Mac OS Extended (Journaled). Type a name for your disk, then click Erase.
- After the drive is erased, close the Disk Utility window.
- If you’re not connected to the Internet, choose a network from the Wi-Fi menu.
- Select the option to Reinstall OS X.
- Click Continue and follow the onscreen instructions to reinstall OS X.
Additional options
If you want to reinstall the version of OS X that came with your Mac, or if you need to reformat your entire startup disk, hold down Command-Option-R at startup instead. This starts your Mac from Internet Recovery.
If you have more than one partition (like Boot Camp) and you want to erase the entire startup disk, you can use Disk Utility to remove partitions or repartition your drive. Start your Mac from Internet Recovery, then use Disk Utility to repartition your startup disk before you erase it. Make sure you back up any important data from all partitions before repartitioning a drive.
How to reinstall OS X on your Mac
Take care.
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Apr 27, 2016 9:04 AM in response to rodion15by Courcoul,rodion15 wrote:
If the OS is loaded into RAM during boot, why can't the system disk be re-partitioned or formatted?
And that argument does not hold, since not all of the OS is loaded to RAM and the system needs to ensure its survival. Plus it needs to have other non-code files open at all times, and those reside on the boot volume.
