Warning #1 - Straight from the MSDN download section..
“Important: If you decide to go back to your previous operating system, you’ll need to reinstall it from the recovery or installation media that came with your PC, which is typically DVD media. If you don’t have recovery media, you might be able to create it from a recovery partition on your PC using software provided by your PC manufacturer. Check the support section of your PC manufacturer’s website for more information. After you install Windows 8, you won’t be able to use the recovery partition on your PC to go back to your previous version of Windows.”
If you install Windows 8 in parallel to Windows 7 or any version, please note that Windows 8 Defragmenter may have an adverse effect on any drive other than the one on which it is installed.
Turn off Windows 8 defrag in Performance Information and Tools > Advanced Tools > Open Disk Defragmenter at your earliest convenience.
I know this because I had installed Windows 8 CP on a separate parallel drive and it caused grief to the point where I questioned the integrity of the hard drive and actually scrapped it. The same thing is happening on a drive that, despite being older, is known to be a good drive.
Note that this might also be an issue with Windows 7 defragmenter adversely affecting Windows 8. Personally, I think that it is a good idea to use 3rd party defragmenters and ensure that the drives holding the operating systems are excluded from each others drive lists..
One other point. Windows 7 will index the Windows 8 drive, so don’t be surprised to see hard drive LED activity for 24 hours or so after installing the Release Preview..