Downloaded Sierra but won't install.

Well, I found a solution for Sierra and High Sierra.

You need a pen drive or external drive to install or new Mac OS X won't install (anymore).

MAKE BACKUP OF YOUR DATA BEFORE PROCEED!

1. Download Sierra or High Sierra from AppStore;

2. Follow this: Create a bootable installer for macOS - Apple Support

3. Use Alt/Option when reboot your Mac and choose the Pen Drive or external drive

4. At this point (YOU WILL LOSE ALL YOUR DATA, MAKE BACKUP BEFORE PROCEED), if you haven't, I suggest you to create 2 partitions on your HD drive, one for data and another to MAC OS. Run Disk Utility and format your HD with HFS journaled (if you have SSD on your system, you can format with APFS)

5. Run Install Mac OS Sierra or High Sierra.

Works like a charm for me!

I tired it but it not working

I've had the devil of a problem with this issue. My iMac crashed. I went through various hoops, all the various boot options, checked the hardware, eventually, finding no other reasonable option, erased the hard disk. It refused to reinstall using any of the reinstall options. Disk locked even though erase parameters were correct. Command line functions didn't work. Installed a boot-up version (of Lion) to an external hard disk .That worked, and reinstalled an O/S to the hard drive. Upgraded to SIerra, but not the latest version. Tried updating to the latest version, which was coming up an 'option' in Apps, and the same original problem occurred: It would stop half way through boot up and refuse to work. Quick-erase and booted up from external drive again and reinstalled the working version of Sierra. Many aps won't install though as they require the newer version of Sierra.

Hi!

Have you tried deleting the installer and then re-downlading?

1. goto the dock
2. click on finder
3. goto the menu bar
4. click on the go menu
5. choose applications
6. find Install macOS Sierra
7. drag macOS Sierra to the trash can
8. go to the menu bar
9. click on the Finder menu
10. choose empty trash (this will delete every thing in your trash can.)
11. reboot
12. open the app store
13. click on sierra
14. click install

please excuse my butting in, but in my case Sierra failed to install, and won't let me restart the computer to the boot drive, it says it can't find info about the disk. The only thing I could open was disk utility. Tried repair on the drive (250G SSD), no prob with overall drive but the boot partition is un-repairable, according to DU. Was working fine all day until this.

Also I can't delete the installer using above method, no menu or dock, just the 'you're screwed' dialog. All I can get to is Disk utility. Going slightly mental...

try holding option key on re-start then selecting original disk

I have the same problem. When the system restarts, try to install sierra and fails. I try to use disk utility, but du doesn't me do nothing: repair, erase partition, create, unmount, etc. Nothing works, cause du tells me that can't unmount a used volume! I can't access my mac mini anymore. I want to downgrade to El Captain, but I can't !

Same problem here, it would be good to find a solution.

Solved for me by using the "install sierra from usb drive" method. The first install failed, but the second worked, I erased and repaired the hd twice before trying the second time.

Same here. It downloaded started to install and a "Install macOS Sierra quit unexpectedly" Deleted once and re-download with same problem. Now what? Mac is on Version 10.9.5

Have you lost all your files erasing and restarting?

Any other solutions for this? Re-downloading did nothing for me.