For some reason my PC had a bad day and decided to temporary die. I restored from a disk image.
After everything seemed back to normal but 1TB of my passport disappeared. Nothing to see in disk management. Minitool sees it as 1TB as well, HD Sentinel the same. Plugged it into a laptop but it shows the same 1TB.
Try testing your hard drive using WD Drive Utilities or Data Lifeguard Diagnostics. If the unit shows as 1TB even on Disk Management with no separate partition you could be facing a very rare type of disk malfunction.