cant access spare partion on vista but can on xp

I have a quad core pc with 640g hd partitioned into 3 2have op systems on them xp pro and vista home prem the third partition is free for my back ups etc. have had no problem accessing backup drive till about a week ago still ok on xp but not on vista it shows under comp management but is not assigned a drive letter when i try to put this in it says that i have to refresh and or restart pc which i have tried but to no avail, all ok in xp comp managemet thou. I even ran easus partition management and it wouldnt read the hard drive at all through vista. Any ideas on how to get partition working again through vista would be greatly appreciated. Pc is 6 months old and i purchased it thru MSY