|It appears the drive is partitioned into one partition but has not been formatted. |
If no partitioning is present, Disk Management would call the space unallocated.
Format using NTFS if the drive will only be used for Windows. You MAY need to also assign a drive letter, if one is not automatically assigned. You can also change the drive letter assignment to a higher, never used letter if the location of the external is going to be important.