Get exact size in bytes of a disk & partitions in windows
Have you tried checking WMI for accurate values? You can query common aliases through the wmic command:
C:\>wmic partition get name,bootable,size,type
C:\>wmic diskdrive get size,status,TotalCylinders,TotalSectors,SCSIPort,SCSITargetId,model
wmic diskdrive : for physical size
wmic logicaldisk : for you know