Whither NiFi directories and files once NiFi running?

[Russell Bateman]

/nifi.properties/ locations, such as 
nifi.content.repository.directory.default, are typically specified as 
relative paths, but I suppose they could be arbitrarily complex. 
Whatever the case, I need to discover the actual path to the content 
subdirectory at runtime not because I want to create, delete, read or 
change files there, but only to know what volume this subdirectory is on 
so that I can use File.getUsableSpace()in a custom processor I'm writing 
that will monitor space left.

Is there a reliable way to discover this, directly or via a property 
using NiFi Expression Language?

Thanks,

Russ