Check in Windows Disk Manager. In the LOWER RIGHT pane you should see all devices connected, whether Windows is using them or not. Look for a block representing the second HDD. It should have a label like "Disk_1" on the left and a size. Beside that there ought to be a block representing the Partition on it that is used as a disk, and that block MUST have a letter name like "D:" or "E:" for Windows to use it. If that block is just called "Unallocated Space" there's another problem. But if it appears to be a valid Partition just missing a name, RIGHT-click on it and Change its Name to one that is not currently in use. Exit out of Disk Manager and reboot to make that change work.