virtualbox.org

Ive attached a differencing disk to a immutable VDI. Hence, the differencing disk never saves any changes. Every change is lost upon reboot.

Why is the differencing disk very big? The snapshot of the vdi is 900MB! It should be zero?

Unless things have changed recently, I believe differences are discarded next time the VM is started. They are NOT discarded when the VM is closed. So yes, the differences file could be that size.