Is there a way to get to a data store located behind a server?

You can't directly capture it, but you can get credit for it.

When you capture all of the diagnostic subroutines in a system, you automatically get access to all the data stores and the hacking mini-game ends. So in the example in the screenshot you posted, if you were to capture the diagnostic subroutine node (the red one), the hacking attempt would end in success, but you would automatically receive whatever bonus was contained in the data store beyond that subroutine (and all the others, too). In some puzzles, going for the diagnostic subroutine(s) is easier than going for the I/O node.

Throughout my play, where my OCD tendencies compelled me to hack every single hackable thing I found, I never ran into a single system where you couldn't get all of the data stores either by traditional means or by hacking the diagnostic subroutines.