Hello, Some news about this topic,
it seems that the length of the folder where I save the c4d file have an incidence on whether the volumeloader work correctly or not ( C4d R23).
if I save the file to E:\data\tests\export\USD, the volume loader does not work, If I save the same thing to E:\USD, it works.
here are the files ( I renamed the files afterwards, tell me if you find a difference ) Untitled2_longfolder.c4d Untitled2_shortfolder.c4dUntitled2_Dense Stratus.vdbUntitled2_Cumulus.vdb
the files' reference are relative so they should load fine. ( at least in a short folder)