Hello Stefan,
Sorry for my late reply.
If you want to run jobs concurrently using the same pool and only one drive (your FileStorage device), your jobs should be configured to use this only pool. They will use only one volume in this pool since you have only one drive.
From your first post, the problem was not about bacula needing to create another volume, but that you had only one drive and your jobs were asking for volumes in two different pools.
About the job that you had problems to cancel, I suppose, because of my delay in replying to you, that you had already solved it. But if you would like to confirm if this is a bug or not you can run your daemons in debug mode: start your daemon(s) in debug mode (./bacula start -d100, for example. The higher this value, more output will be produced) or configure debug mode using the "setdebug" bconsole command.
Best regards,
Ana