You probably have RAID 0 configured for your hard drives.
This means that the pc will see the hard drives as 1 hard drive.
For example:
I have 2 hard drives and 1 game installed that has 6 parts
What happens with RAID 0 is the following
Parts 1, 3 and 5 will be saved to hard drive 1
Parts 2, 4 and 6 will be saved to hard drive 2
RAID 0 is also known as striping, it is very efficient for boosting the performance.
But, when 1 hard drive fails, you loose the data that is on the hard drive and this makes programs unusable.