If Windows installer can't find either a partition it can install to or unpartitioned space on the drive it'll appear as if it doesn't even find a drive.
If the drive has been partitioned under another OS, and a FAT or NTFS partition isn't the type created......
well i have unallocated space set aside (about 70 gigs) and it still doesn't recognize it. I tried formating it (with ubuntu) to NTFS and it still won't recognize it.