He set up a Raid 0 (Stripe)
The hard drive is still there, and you are using it.
The 2 drives are working as one drive.
Stripe Array can improve performance because both drives are working at the same time to read/write data across them.
For more info,...
Standard RAID levels - Wikipedia, the free encyclopedia
The down side is, if one drive goes bad, you lose everything stored there.
Always keep a backup of your data off the Stripe Array.
The only way to fix this is,,,,,,,,, Back up your data.
Break the array and reinstall windows from scratch.
Also note,, that using 2 disks of non-equal value will result in an array of the smaller disk.
Which is why you are not seeing the full 2Tb of the second drive.