My first guess is that these are new drives and need to have partitions created; a new partitionless drive will definitely not show up in "my computer."
To help us get beyond a first guess, you will have to provide more information, such as what OS you are running, whether or not you have used these disks before, and how they show under Disk Management (control panel - administrative tools - computer management). This way, we will know where the drives are and if they are partitioned.
PS - if my first guess is correct, you can open Disk Management as described above, select the new drives, and create one or more partitions on each. Partitioning disks is a kind of religious question - some will create one large partition, somewill create several smaller ones, some will short-stroke, and some will puzzle me entirely.
Scroll down to the BOTTOM of the middle pane, find your new drive, and right-click on it to create a volume (this partitions it) and then again to format it. For a separate data drive you probably just want to create one partition (volume) that fills the whole disk.