Cannot take a snapshot when there is a Writethrough hard disk attached

[nvivo]

Trying to take a snapshot of a VM that has a disk in writethrough mode (even if you have other regular disks) causes the GUI to show an error:

Cannot take a snapshot when there is a Writethrough hard disk attached ('/path/to/disk.vdi').

Result Code: 
0x80004005
Component: 
Machine
Interface: 
IMachine {31f7169f-14da-4c55-8cb6-a3665186e35e}
Callee: 
IConsole {1dea5c4b-0753-4193-b909-22330f64ec45}

This makes it hard to have a "system disk" that you want to snapshot and a "data disk" where you persist documents, settings and other stuff. The workaround today is to manually remove the disk from the VM using the settings window, take the snapshot, then re-add the disk. (And you must remember everytime you restore it).

In VMWare, using the "persistent disk" that is the writethrough equivalent, the behaviour is to take the snapshot of regular disks and ignore the persistent ones.

So, VBoxManage could have a similar behaviour, and maybe show a simple message like this when taking/restoring snapshots:

The following disks are beign ignored because they are in writethrough mode:

/path/to/disk1.vdi
/path/to/disk2.vdi