How do I upgrade if I can’t use the main hard disk?

My son recently tried to reactivate his World of Warcraft account so that he could play the new expansion. Every time he tried to log in, it said that the servers were full and when I eventually called Blizzard customer support, they said that this was because we were not fully upgraded (I’m using Snow Leopard). I’ve tried to download Mavericks from the App Store but every time it tells me that ‘This disk cannot be used to start up your computer’. What can I do? I only have one disk! Claire Theobald The ‘server full’ message is unhelpfully cryptic, but it seems the World of Warcraft launcher was changed back at the time of the previous expansion (Mists of Pandaria) and now requires OS X 10.7 (Lion) or better. This normally ought not to be a problem for you. Mavericks is free and upgrading from Snow Leopard is just a matter of hitting the ‘Upgrade’ button on the App Store. It’s likely not to have worked for you because the partition table is slightly corrupt. This could indicate a faulty disk, or it could just be ‘one of those things’. Happily, there’s a simple fix. Boot from the original Snow Leopard install disk (restart with held down) and open Disk Utility from the installation menu there. Select your startup disk, click the Partition tab and drag the resize handle in the bottom-right of the rectangle that shows the partition layout. Shrink the partition by a small amount – anything over 128MB. Don’t shrink it into the blue section of the partition or you’ll lose data. Click Apply, restart and try installing Mavericks again. Then reboot in Recovery mode (hold down ç+R while restarting) and use Disk Utility to move the partition boundary back so that it fills the disk again.

C