The hard disk with data on it appears to be unallocated.
What may make a hard disk become unallocated?
Do you know how to fix an unallocated hard disk?
I guess unallocated means the memory does not belong any disk partition. You can use some partition tool to like GParted to make the memory part of a partition. You might have to create a new partition if no partition exists. I think Windows 7 has a build in partition tool. I also know that it's possible if you boot from a Windows XP CD.
I found it was because I installed several disk cleaning software, the windows couldn't find the disk.
Uninstalled the software fixed it!
i use tweaknow powerpack. been using it for more than 10 years, maybe since windows 95 to clean my registry. dunno if it really clean it and make windows better, but no problem since i start using it.