Thanks a lot for your answer - always good to take a step behind and look for completely other solutions.
I am writing a plugin for a monitoring-system running on Linux servers. So Powershell is not an option. But I wonder how it could be so easy in Powershell? I always thought these SDKs are just wrappers around the generic XML-based API. That was at least the impression I got when working with the Perl ZAPI.
If you could point me to the right (Z)API I would be really grateful.
Thanks so far, Ingo
P.S. Just read your answer again: The problem is, that I can not use the container-type as indication for a spare disk. In the meantime my assumption, that we can use the the type field a indicator proved as wrong. So back to the start ... I will file a new question, once we have found the right attribute.