Not sure when it started, but I can no longer open a bak file. Here is an example of the message I get "There is no application set to open the document “Can You Sleep At Night- Master-02.bak”. Search the App Store for an application that can open this document, or choose an existing application on your computer.
Buck
Maybe change the “.bak” to “.cpr” and it will work?
In Mac, get info on the item and you’ll see a pop up menu titled “Open with:” Set it to Cubase, and also click change all.
It’d actually be possible for Steinberg to invent a unique extension for their backup files and then associate them with Cubase so they open straight away… Then we wouldn’t have to change the extension. Maybe a nice feature request? Also it’d be fantastic if they put them all in a subdir called Backups to keep the top level tidy!! I’m sure this has been requested many times.
Mike.
On Mac ‘bak’ does get associated with Cubase on installation. In this case the there is some issue in the Mac Launch Services database, (such as another application also using .bak or Launch Services files being corrupted) and it needs to be reset manually. So yes, maybe something like .cprbak would be better.
While there are different reasons that could happen, the fix is manually setting
it, or rebuilding the Launch Services database, which is more info than this
thread needs imo… 
It is discussed at https://discussions.apple.com/message/23607344#23607344 though…