Expand left data inaccessible

I have swapped 1TB disks in my N5200 raid5 configuration with Seagate 1.5TB ones (one by one). After all 5 disks where rebuild, I proceeded to expand raid to gain extra space.
The expand process completed in 30 min or so and probably failed since it left inaccessible 2TB of data space (see picture).
I run rebuild/expand using firmware 2.00.18.2. Now I cannot do another expand and my accessible data space is the same as with 1TB drives.
I have downgraded to 2.00.15 and 2.00.16, but it doesn't help.
Anybody has an idea how to regain those 2TB space?

This is the picture URL (I am not allowed to insert link directly):

mndc.com/picture1.jpg

Thanks in adavnce,
David