To expand the VM’s hard drive, make sure you have enough free space in your physical harddrive to commodate the old file + the new file.
e.g: to expand the virtual hard drive from 80GB to 100GB, you need 100GB free space in the hard drive story the VD, not 20GB, but 100GB.
This is because VMWare will create a new file with the size of 100GB and copy the content from the 80GB VD over, then delete the 80GB VD