Adding partition to existing USB drive (data only)

[allen meyers]

My existing data drive is working great, however I want to add a
partition (which I know how to do)
My purpose is categorizing information, i.e, moving from one partition
to other.
My question is two fold when that icon comes up on the desktop is the
entire drive mounted or must I mount the partitions separate?
Then at that point will I have two browser windows one for each
partition and its then a drag and drop process.

Thanks 

Allen