Convert Dynamic to Fixed size VHD

Hi,

When I convert a dynamic VHD to Fixed size I get an error message.  The disk is on a SAN volume (200GB).  The VHD is a 21GB file.  There are no snapshots involved.

If I choose to convert the file and select the same file to convert I get:  The file exists.  SO..  

If I choose to convert the file and select the a different file name I get:  There is not enough space on the disk.

There is plenty of SAN storage, so do I need to expand or compact the disk prior to conversion?

Any advice would be great, thanks Matt.