OSX El Capitan Reboot Problem

Try resetting the PRAM per Intel iMac SMC and PRAM resets

Resetting your Mac's PRAM and NVRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

Restart the computer into Safe Mode then restart normally.

Reinstall El Capitan Without Erasing the Drive

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.

Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the indented Macintosh HD entry from the the left side list. Click on the First Aid button in the toolbar. Wait until the operation completes, then quit Disk Utility and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Continue button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless and more reliable.

I suspect that the drive is either corrupted and needs to be reformatted or it has failed. I hope you have a backup of your files. If it's the former:

Install or Reinstall OS X from Scratch

Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.

Erase the hard drive:

1. Select Disk Utility from the main menu and click on the Continue button.

2. After DU loads select your startup volume (usually Macintosh HD) from the

left side list. Click on the Erase tab in the DU main window.

3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on

the Security button and set the Zero Data option to one-pass. Click on

the Erase button and wait until the process has completed.

4. Quit DU and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Install button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible

because it is three times faster than wireless.

This should install the version of OS X that you had installed.

Please make an appointment at the Apple Retail Store - Genius Bar and take it in for service. If you are not in the USA or cannot set up an appointment this way, then contact your local Apple Store directly.

I have this problem and have tried all the solutions suggested without success.

I can't reboot into safe mode, permissions repair has disappeared from Disk Utility, but can reload the OS which takes time. I've done it 3 or 4 times now.

Other support sites seem to think it could be an "extension" from apps that haven't been updated to El Capitan yet.

I'm going to leave my iMac running to see if Apple do an update.

Could revert to Yosemite from Time Machine but am reluctant to do that.