I noticed some small print on support page:

NOTE: Capacity expansion is only possible on RAID 0, 1, 5 and 6. Please keep in mind, there should only be "1 virtual drive" on the array. If you have more than 1 virtual drive, then you cannot enlarge your array. You will have to create a backup, delete the array, create a new RAID and restore your backup to the new virtual drive.

Is this noticed anywhere else? (I didn't find it!).

I have the exactly same "no go" configuration with RAID5 array and 2 VDs: 20GB for OpenE OS and 5TB datastore which I try to expand to 6TB.

So, I have to:
- create "all new" system with separated System HDD and larger virtual drive.
- prepare it for replication from Active node
- replicate (sync) all data
- switch active/pasive nodes
- repeat the exercise on 2nd node

Any better idea?
Is there any utility to "migrate" DSS7 system VD to separate HDD?