probably you have one physical drive but been partitioned to two virtual drives.
some do that to separate the OS drive from the data drive.so if you had reinstall and format the OS drive you wont lose the Data on the other drive
Most (almost all for the last decade) prebuilt computers use the D partition to store the windows and system files necessary to do a factory restore. If this I'd the case you don't want to mess with it or putting anything on that partition