Current Data lost, everything back to last snapshot?

[mpack]

If you pointed CloneVDI at the latest snapshot, as instructed, then there are no missing files - else CloneVDI would certainly have mentioned it, and could not have created a clone.

[MTSE]

There are 4 snapshot VDI in the snapshot directory, including the one I deleted and then recovered via recycle bin, and I did the clone on what I believe as the latest snapshot. CloneVDI ran successfully and created a cloned VDI with thesize of 250MB or so. I then create a new VM as instructed but the VM failed to start.
I then tried to use CloneVDI on the other VDI and they all ran successfully and produced various cloned VDI. Two of them have the same size. The recovered VDI has the bigger size of 400MB. None of these can start the VM.
Am I missing something? Thanks for your effort trying to help me. It is much appreciated.

[mpack]

Please expand on "fails to start". That encompasses a great many possible behaviours.

[MTSE]

It says it can't find any operating system.
My setup uses univention server to run owncloud.
I have also tried to start a clean Linux and attach the cloned snapshot VDI, but it shown as unallocated partition.
The 7-zip failed as I don't have enough space on hard disk. I need to find a bigger hard disk first. Although the size on disk is only 250MB after compact, when 7-zip trying to read it, it needs the original allocated size of 2T.
My mistake of deleting the attached VDI is a costly one.

[mpack]

My mistake of deleting the attached VDI is a costly one.

Accidents happen. The real mistake is not having a backup.

Can you check if your original VM had EFI enabled? Did you also enable it in the new VM? If the answers are yes and no then it would have the effect of making the drive unreadable.

Is there any possibility that Univention Server uses encrypted disks or partitions? That could have a similar effect.

As to deleting the snapshot VDI: provided you did it using the GUI then running out of space should not have been a fatal error. VirtualBox would first create a merged VDI (merging changes from the deleted snapshot back into a neighbor). Only if that succeeds would VirtualBox fix up the connections and delete the old elements. That said, experienced users (a) avoid snapshots and (b) make actual backups.

I'm not sure what you're saying about 7zip. 7zip should allow you to extract individual files, I doubt you have any that are 2TB. This is why I started thinking encryption - but I'm not sure what that would look like on Debian.

[MTSE]

EFI was enabled. Owncloud runs on Univention server and uses a database rather than flat files. So, I am afriad it is rather difficult to get back what I lost.

For 7-zip, it needs to read the whole VDI and temporary stored somewhere before it shows you content. I didn't have enough space before. Now, I managed to find some space but the open of the file failed. It reported as corrupted.

A costly lesson earned, I should have used proper backup and don't play with snapshots.

Again, mpack, thanks for your help. At least I understand the Virtualbox a lot more now.