Extent the capacity of My Cloud

The following is my situation: about a year ago my device (WD cloud mirror 2x 3TB) was struck by lightning and the network interface (this was connected to the router, which actually was hit by the lightning) on my WD drive was defective - both disks were still 100%.
Insurance replaced the device, but due to the fact, that they couldn’t replace it with the same - they send me a WD cloud mirror with 2x 6TB.
I have plugged in one of the 3TB drives and wanted a rebuilt - it worked so far, that it copied a 3TB partition with all data on to the 6TB disk. Then is swapped the 3TB disk against the second 6TB disk and a mirror was created. Now I have on a 2x 6TB system still only a capacity of 3TB available for My Cloud Mirror.
My question: how can I release the 6TB to the mirrored device? Thanks for your help.

Hi daspack,

I would suggest you to contact WD support for device capacity issue.