Hard drive letters after C: are pushed ahead when a second drive is attached
Cause: This generally happens when the second drive has a primary partition on it. Since primary partitions take precedence over extended ones, it will take up a drive letter immediately after the primary partition of the first drive, i.e., C:.
Solution: After installing the second drive, make sure the drive is formatted and that all partitions created are extended. If the second drive contains data, move the data to the first drive and then remove all the partitions before creating new (extended) ones.