Problem with online snapshots and writethrough mode

[Phylon2012]

Hello,

I am running Kubuntu 12.04 on a W7 host (using Vbox 4.1.22), guest additions installed. 2 virtual disks are attached to the guest, one of them being in writethrough mode.
When I am reverting to a previous online snapshot, my files I created during the last session are gone (can't see them with ls on the writethrough mode drive).
After searching the forum I found that this issue also came up with windows guests in 2010, but seemingly without a real solution.
After looking into it further, I found that I do not have the problem with offline snapshots. Additionally, I found after reverting to a previous online
snapshot, that unmounting and then mounting the wtm-drive again will bring back my files in this case.
Is there still something else to configure somewhere to make this drive seamlessly work?
Or is it a problem with the guest additions, may be some kind of an malfunctioning update process when starting from an online snapshot?

Thanks in advance

Edit: I just want to add that I found that the "invisible" files are overwritten (at least the metadata) when I create new files on the wtm-drive before
unmounting/mounting as described above.