OP, you may not know where to look. Click on Start at lower left, RIGHT-click on My Computer, and choose Manage. In its left menu pane click on Disk Management under Storage.
The right large pane will be subdivided into two panes. The upper pane shows you all the devices Windows can use now, and those are the only ones you will see in My Computer. Concentrate on the LOWER RIGHT pane. It SCROLLS so you can see all it has. Each storage device is represented by one horizontal block in which there is some info at the left end: a unit label like "Disk 0", a disk type, a size in GB, and a status. To the right you find one or more sub-blocks, each representing one Partition on the unit. An optical drive may have no Partitions, especially if there is no disk in it. This window will show you all the valid hardware devices so you should see EVERY item that is showing in BIOS Setup.
Scroll this window and you will see your SSD there, as well as any optical drive or HDD. IF the HDD you have is a new empty one, it will have only one major sub-block labelled "Unallocated Space". If that's what you see, you need to prepare the HDD for Windows' use. You should RIGHT-click on that space and choose from the little menu to Partition or Initialize the unit. It will offer you some default options, one of which will be the size of the Partition to Create. (A Partition is just one chunk of HDD space that will be treated as a "drive" by Windows.) Many people want ALL of the space on the HDD in its only Partition; others want to split the HDD into two or more and have them as separate "drives". It's your choice. You do NOT need this unit to be bootable since you are booting from your SSD. Most other options will be set correctly by default. When you're happy with those settings, tell it to proceed. IF you are using an older version like an early Win XP, you MAY have to do a separate step after this to Format the HDD, but most current versions do that as part of the process. It will probably take a LONG time (many hours) to do it. When it's done, back out of Disk Management and reboot your machine. The HDD should show up now in My Computer ready to use.
On the other hand, IF that HDD already has data on it from previous use, you will see at least one sub-block identified as a "drive" with further info in it: a disk name like "Harrys Disk" or something. the letter name Windows uses like "E:" (your SSD will be "C:"), a Partition size in GB, a File System like "NTFS", and a status note. If that is what you see, look closely at the end of the first line: does it have a letter name like "E:"? If not, that is why Windows cannot use it yet. RIGHT-click on that sub-block and choose the Change the letter assignment, then choose a letter name for it. Back out of Disk Management and reboot for this to take effect.
Another possibility for a used HDD containing data MIGHT be that the File System shown is "RAW". That usually means there is minor corruption of a some data in the system files at the start of the HDD, but almost all your old data is still there. In that case IF you want to recover that old data there are particular techniques and tools you will need. So, if that's what you find, do NOT make any changes to the HDD. Just post back here and we can advise further.
Lat us know how things go.