Hello when you log on a Surface Hub with your O365 account and start a collaboration session with the Whiteboard, it create an .wbx file in your OD4B under a folder name "WhiteBoard App Data" and provide you with a special link you can paste in a Skype Meeting or keep in your Team in order to continue the white board session later.
Question is: Now how can I open those file in the Surface Hub? Yes if I keep in a safe location the link provided when using the collaboration sessions I'm fine, but what if I want to move the file in my Microsoft Teams Team or SharePoint location and re-open it later.
The WBX file I believe is just a storage medium for containing the Whiteboard data for the last session. It's not meant to be usable for the end user, and you can't use it to reopen a previous collaboration session.
When exiting a collaboration session on the current version of MS Whiteboard, it does remind you to save your Whiteboard to OneDrive or via email otherwise all data will be lost.
As @Steven Collier has mentioned, try using the new Whiteboard Preview app which allows multiple whiteboards to persist under your O365 user account instead if you require this kind of functionality.
This is not entirely true, if you save the link, whether to a Teams URL tab or Word doc you can access the whiteboard later via that link from a Surface Hub signed in on the current Whiteboard app. If you delete the file from your OneDrive the link will no longer work. The file that is in OneDrive cannot be directly opened without the link... By design it would seem.