I wonder if it's what happened to me a couple of years ago while reinstalling XP. Two of my hard disks appeared as unallocated, and with capacities like 30MB or 723MB instead of 1TB. I fixed it with this (free) tool:http://blog.atola.com/restoring-factory-hard-drive-capacity/
Don't try formatting your disk. I did that with one of my disks. The tool restored the disk's capacity but I lost the files on it. On the other drive, after running the tool, all my files were still fine.
I did find a page, eventually, somewhere on Microsoft's site, recommending that you disconnect all drives before installing Windows, leaving only the drive where you wish to install, of course. Too bad I read that AFTER the fact