XigmaNAS

I am using nas4free version 9.1.0.1.
I have 6 Seagate NAS drives 4TB each configured as ZFS RaidZ2
Unfortunately 2 drives were damaged at once. The ZFS raid is still working in DEGRATED mode
I have one spare drive available.
Disks=>Management shows it as:
da4 3815448MB ATA ST4000VN000-1H41 SC43 ST4000VN000-1H4168 Z3011YZS Always on Unknown or unformatted ONLINE
I tried to format if, but Disks=>Format does not show it and thus I can't format it.

Questions:
1) Am my missing something? Why can't I select it for formating?
2) May this be a bug? Should I upgrade my NAS to newer version?
3) In that case, should I upgrade to a version 9.x or 10.x series?
4) Is it safe to try to upgrade nas4free whyle still in DEGRADED ZFS state?

Thanks in advance for any help provided.

I am currently following these instructions: FAQ 149: http://wiki.nas4free.org/doku.php?id=faq:0149

When I posted my request I believed that the ZFS control was entirely done on the nas4free Web interface.

The new drive is currently being resilvered at 310MB/s and estimated 6h30min to resilver 7.1TB of data

I will post the results as soon as possible.

After 9 hours it is still resilvering at only half the speed it started with:

zpool status gives:
scan: resilver in progress since Tue Jun 28 00:36:52 2016
3.96T scanned out of 7.09T at 130M/s, 6h59m to go
675G resilvered, 55.85% done

Is it normal to have such a slow speed? Is there anything that I should check?

It completed the resilvering process in 13 hours with no errors. Speed was around 130M/s