vpn connection

I am running windows server 2003 with vpn connection I have 3 drives mapped on all computers running xp pro “f drive, u drive, w drive” one of the computers is used to access the vpn often and the f drive will sometimes not allow access and the other 2 drives will it will not let you disconnect the drive so you end up having to map a new drive to g or h to get it to work then you can disconnect the f drive and then remap it. Any help would be greatly appricated.