It means: In a snapshot chain each new file stores only those sectors which it thinks have changed since the new file was created. If you go in and modify an early member of the chain then it effectively corrupts every newer file in the chain, because the newer files have no knowledge of the change you made. Also VirtualBox has a "modification UUID" record to detect if older members have been modified improperly, and you may find that it then refuses to work even if the corruption would be recoverable.johnbot wrote:it gives you a warning that "to modify this vdi file will cause the upper layer snapshot does not available!" - Whatever that means
Avoid snapshots (use full clones instead) and problems like these do not arise.