Be aware that Windows 8 has a modified NTFS file system.
I have two hard drives. W7 on one and I installed W8 on the other. Rather than dual boot I just selected the drive I wanted at boot up. This means that you can see both systems in windows explorer and easily copy files across to the new system.
BUT and there's always a BUT!
After installing W8 on one drive with the W7 drive disconnected, just in case, I rebooted my system with both drives connected and selected the W7 drive to boot and while it was doing so I put the kettle on. When I got back the computer had noticed the modified file system on on the W8 drive and assumed it was corrupt. So it tried to repair it - I am now re-installing W8 again - carefully!