Fixed VHD with no free space

Hi,

I've converted a 38GB VHD to fixed, however the disk size is now 100GB with 5GB available free space?  This server will have additional software installed so I need further space in future.

The VHD disk is on a SAN.  What I want is the fixed size disk to have "disk space".  Am I missing something here with fixed size or have I done something incorrect with the process?

Thanks, Matt.