You basically need to re-fetch the mission details before each group is processed, and filter out those already with an assigned drive. This can be placed in a second scenario.
So one main scenario to fetch all the groups, for each group trigger your second scenario, and wait 5 mins.
When your second scenario is triggered, if there are no missions that pass the filter, then nothing happens.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!