For work, I recently had to expand a system disk for a VMware VM.  Of course, the conventional wisdom is that you cannot expand the system disk.  But did I let something like common sense stop me?  No!

Basically, I converted the VM.  The VMware converter allows you to P2V (physical to virtual) and V2V (virtual to virtual) computer systems (mostly servers).  But, one option allows you to convert one machine between ESX hosts.  During the conversion process, you can state how large you want the disk to be.  Once at that point, you can expand the size of the disk.  Afterwards you can run the vm with the completely expanded disk.  I'll post a more detailed walkthrough later.