Check if you VM has snapshot... and if Yes, remove the snapshot and try expand the disk again.
Always confirm the following before increasing the size of the virtual disk:
You don't need power off the VM to increase the disk, removing snapshot is enough to be able to increase the vdisk
Yes you are right no need to power off the VM.
Must delete the snapshot or consolidate it first before extending the disk size.
Windows Server 2003 need a reboot if System Disk (c:\) is being extended.
Above than 2003 doesnt require a reboot. only extend command is enough to extend the disk . Doesn't matter if disk is os or data.
You can use dell's extpart to extend 2003 c:\ with out restarts