Need some advice re Shares and Drive Mapping

we have a ReadyNAS Duo working perfectly for many years, we probably use it very differently to most people, in that anyone at the office (who is on the LAN), can map a network drive to the NAS (all our work laptops are Windows OS) , when they arrive at the office they backup their docs to the NAS, they do this by simply dragging a folder on the laptop to the Z drive on the NAS (they create a directory on the laptop which is named after the person eg johnsmiith, so we all know whose files are whose) - everyone very happy including me ( the office admin guy )

up to this time there has only been one share on the NAS called "backup"

now I want to provide 1 person with his own personal share that is password protected (nobody else can access these files) and map it say to the Y drive on their laptop, so I created a new share called "bobsbackup" , \\readynas\bobsbackup but for whatever reason windows does not allow me to map this to the next current drive letter available Y on hos Windows XP laptop

he can see the Z drive fine, and drag docs to it

but adding another share mapped to Y seems to be beyond me, can I ask for help in how to achieve this in a step by step process please, the manual for the Duo is not helping me one dot.