if you are 100% sure that you are in the correct drive and it boots ok without the other drive connected,
then all you need to do is go to disk management and format the 'spare' drive
all the tools you need are already in windows to do this
or you can delete the partition on the 'spare' and create new partitions - you can have a maximum of 4 logical partitions OR 3 logical partitions + an extended partition with 3 partitions (basically 6 usable partitions),
keeps all your stuff in order - but that, maybe, is for another day
but - if you are not confidant enough then someone will walk you through the process, just ask