Sorry to bump this thread now, but I just had the exact same problem with OMV 4.1.3-1 (Arrakis). For me, the correct code in the shell file was THIS:
and NOT the following:
Please have a look into this thread: Not enough disk space, post no. 3
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dfree command = /usr/local/bin/dfree
Notice the changes from $3 --> $2 and $5 --> $4
Hope this helps someone.