Q: MacBook not installing Yosemite OS X

how much of free space on your drive you have?

check your drive and repair permissions

also you can try to install from bootable usb drive

hi, same person different account here. How do I do that? it seems that I can't actually get out of he screen that I'm on.

post installation log here

Press the power button hold unitil it switches off, restart, do it again.

How? Isn't it just the coding in the background of that photo

i've tried, goes back to the same page

So I take it you bought this second hand Mac that was working with some version of Mavericks but you never really wiped it out and started fresh before attempting to upgrade to Yosemite?  I would highly recommend that you perform a clean installation which means wiping out the internal disk completely and installing Yosemite from an external flash drive.  Now for the potential gotcha.  Your Mac might not support Internet Recovery, if it does, you most likely need to install a firmware update. Possible that the firmware update was already installed.  See this list:

Computers that can be upgraded to use OS X Internet Recovery - Apple Support

Hold CMD+Option+R while powering on the Mac.  You should see a spinning globe.  That is Internet Recovery Mode.  Now you are going to need a fast Internet connection to do this and even then it will take a long time.  Basically, this Internet Recovery mode is running in the Mac's firmware and will let you connect to Wifi or Ethernet and get online. Then it will download a new recovery partition to your disk and boot from that to download the rest of the operating system installation. 

Once you get it to boot to the installation you'll need to click on the Utilities menu and Disk Utility then repartition the internal had disk to a single partition.  Then exit disk utility, reboot if it says to and re-install.  Again it will take a long long time as it needs to download everything. 

Alternatively, if you have access to another Mac you can create a bootable flash drive for Yosemite and use that to boot the broken Mac, repartition the disk and re-install.  This method is faster as it is quicker than the Internet download method. 

http://www.macworld.com/article/2367748/how-to-make-a-bootable-os-x-10-10-yosemi te-install-drive.html

Thank you! I'll try this when im on a better internet connection. to add to the mess I've made, I haven't backed everything up. Its not a massive problem as I didn't have too much on there already, but it would still be annoying to lose. Any way to recover it?

James,

she has Mavericks (latest) already, and thus a RecoveryPartition. Her mac is perfectly fit to run Yosemite. See the first post.

A clean install is a good idea, she has to have backups then.

CMD+R+Alt(option) will go to the internet recovery, which will install the original OSX, which is SnowLeopard, SL has no RecoveryPartition ! And thus not  an advisable option.

She better goes to CMD+R for the actual software, Mavericks. Then reinstall Mavericks. Or from a bootable stick with mavericks on it.

Lex

if my Mac is perfectly fit to run Yosemite then why didn't it?

Read my post to James to see if it is wise to follow that.

- Because something went wrong. And we are trying to find out what.

- Much older (years) macs already are fit for Yosemite.

- Standard install works like this: download the new OS, click install, it is doing some preliminary work, then the mac restarts, during the restart the new OS is really installed, when everything is OK, the old system files and the new installer are removed.

When it does not complete, the old OS is still there and theoretically should run after a restart. When something goes wrong in the latest part the old OS may be damaged too.

- The old OS is Snow Leopard, no Recovery Partition.