When you get following message in the Boot Camp Assistant:
"The startup disk does not have enough space to be partitioned. You must have at least 10 GB of free space available."
It means that the Boot Camp reserves 20 GB for Windows partition and then it needs additional 10 GB of free space to run Mac Os X. Hence you need 30 GB of free space to install Windows and not 10 GB as the message could mislead you.
And there is one more hiccup. That 20 GB for Windows must be continuous. Hence if you have more than 30 GB of free space and you still get this message:
- Boot Mac from the install disk (hold the C key as you boot until the language selection screen appears.)
- From the Utilities menu on the install screen choose Disk Utility, then Repair Disk.