It might not be finding it because it doesn't recognize the partitions. We're going to diskpart it, wipe it clean, then reformat it.
Open up a CLI terminal and run with administrator privileges (assuming Win 7):
Hit the 'start' button and type in cmd in the text box - it will pull up cmd.exe up top, right click it, select 'run as administrator'.
This opens up your command box with admin privileges.
Type 'diskpart' and hit enter, this starts the disk partition interface.
type 'list disk' - this lists your disks, starting with 0 as the 'c' drive. Determine which numbered disk is the one you want to wipe. For our example here we'll assume that disk 0 is your C: drive where windows is installed, disk 1 is the unreadable disk.
type 'select disk 1' and hit enter. (this selects the disk)
type 'clean' and hit enter. (this erases all information on the disk)
type 'create partition primary' and hit enter. (this creates a standard primary partition)
type 'assign' and hit enter. (this assigns a drive letter to the disk and registers it in your computer).
type 'exit' and hit enter (this exits the diskpart utility)
type 'exit' and hit enter (this exits the command line interface box).
You should then be able to see it in 'my computer' where you can then format it to whatever specs you want/need.