Yesterday I try to install Windows 8 RTM on my laptop as a second OS (Dual boot with Windows 7), so I chose custom install. After selecting the drive I would like to use and clicking next, I receive the following error:
We couldn't create a new partition or locate an existing one. For more information, see the Setup log files.
When I encountered this error message I have three external HDD, a SD Card and one USB Thumb Drive / Flash Drive connected to my laptop. So I remove SD card and disconnect all external HDD from USB port, only one thumb drive connected to USB port because it contains the Windows 8 Setup files. Then I ran the setup again, finally I can install Windows 8* seamlessly. Hope this helps!
*It worked with Windows 8.1 and Windows 10 too!