I recently read a post by Duncan Epping over at Yellow Bricks where he tried to resize the VMDK of a Windows XP VM running on ESX\vSphere. When he used the VI Client to try and resize the VMDK he was actually unable to because it was ghosted out. By coincidence I had experienced the same problem a week before Duncan after building 5 Windows XP VMs for stress testing of a new SharePoint website. As Duncan mentioned, the only other time I had seen this happen was when a snapshot existed for the disk that needed to be resized. Quickly checking the VM's settings I noticed that by default, when you create a Virtual Machine using the 'Microsoft Windows XP Professional..." Guest Operating System container it actually attaches an IDE disk rather than a SCSI disk (Server 2003 and 2008 Guest Operating System containers use a SCSI disk by default.)
When I first started using VMware ESX I was testing VMware Consolidated Backup (VCB) to dump my virtual machines to a staging area before being copied offsite. I noticed that one of the VMs had a VMDK attached that as far as the OS (Windows Server 2003) was concerned with had all of the data deleted on that volume. After initiating a VCB dump (backup) on the VM I noticed that the VMDK that was dumped to my staging area strangely appeared to contain data. The VMDK size was considerably larger than it should have been for a disk that Windows reported as containing zero data.
Every now and then I need to resize (usually extend/enlarge) a disk attached to a Virtual Machine. I have tried several methods to do this over the years (including combinations of VMware Converter, third party partition manager apps, diskpart etc) but none have been as efficient as the method I discovered during recent VMware training for my VCP4 exam.