Is the option still greyed out when the desktop is powered off?
If so, I would keep it powered off, go into the settings of another VM, add a hard disk, and follow the wizard, when the option to create a new one comes up, choose the option to use an existing disk, navigate to your individual one and select.
Go into this VM settings, increase the disk space and apply. Open this VM, power on, open disk management and see the new disk. Use the option to expand.
If it won't let you, open a command prompt. Run 'diskpart' then run 'list volumes'.
Identify the volume you want to expand eg 'select volume 2'
run 'expand'.
Then exit. Power off this VM. Edit its settings, remove the hard disk (make sure you don't select the option here to 'delete from disk'.
Then power on the original Individual Virtual Desktop.
It should be expand.
Hope this helps,
Dan
