If you can't access all your storage, you should first check if the hard drive has been partitioned. You can do this by entering Control Panel: navigate to Administration Tools and start Computer Management. Then use the tree menu on the left and choose Local Disk Management. On the next page, you will see all your partitions and drives. Check and see if there is a partition with almost 1 TB of space, right click it and choose a letter.
The new drive will appear in your My Computer drives section.