Used this basic procedure to add a hard drive in Windows 2012 R2 Core:

List the disks to ensure you select the correct one:

DISKPART
DISKPART> LIST DISK

Select the correct disk:

DISKPART> SELECT DISK=1

Put the disk online:

DISKPART> ONLINE DISK

Examine the disk attributes:

DISKPART> ATTRIBUTES DISK

Remove the READONLY attribute if on:

DISKPART> ATTRIBUTES DISK CLEAR READONLY

Create a primary partition on the drive:

DISKPART> CREATE PARTITION PRIMARY

Format it:

DISKPART> FORMAT

Assign the desired letter to the drive:

DISKPART> ASSIGN LETTER=D