How to format an unallocated disk in Windows.
Step 1: Type "disk manager" into the Type here to search box in the bottom left-hand corner of your screen, and choose to Create and format hard disk partitions.
Step 2: Right-click on your disk that is showing as unallocated and choose Format...
Step 3: Click OK to format the drive. Give the drive a suitable name by changing the Volume label. Leave the rest of the settings as their defaults. 
The drive will now be visible for you in file explorer.