I've been running a SHR with 4 x 4TB disks with DSM 5.1 and tried to expand it with a fifths 4 TB disk. The DSM told me that the SHR ist expandable and when I started the expansion it started working for about 30 hours. At the end the DSM told me, that the expansion has been finished well and the SHR had all 5 disks included. Unfortunately the total diskspace shown by the DSM was the same as with the 4 disk SHR before!
So I tried to reboot the system and when restarting, the DSM crashed (DSM didn't boot up and in the Synology Assistant it was shown as "Installable"). I switched off the system completely and restarted it again. Now the DSM booted up completely and the SHR was shown with the status "normal" including only 4 disks and telling me that is would be expandable with the fifths disk. Ok I thought and I tried it again to expand the SHR. But this time the expansion was just running for a few seconds and then stopping without a message.
The next step I did was to downsize the DSM to Version 5.0 by resetting all configuration data but keeping the SHR and the data. Once finished with the downgrade, the DSM stepped into the alarm-mode with the first boot up, showing me that I have a broken 5-disk SHR!!!
So I started the repair and after running many hours again, the DSM finished with a working 5-disk SHR, showing up all the diskspace you would expect by a 5x4TB SHR!
The result for me is, that the DSM 5.1 had expanded the 4-disk SHR to a 5-disk SHR on data-basis but something went wrong on the configuration basis (the data is distributed to 5-disks as it should be on a 5 disk SHR, but the config data of the DSM says, it is a well working 4 disk SHR) - very dangerous for your data! The interesting thing is, that DSM 5.0 does recognize this error and is able to correct it.
I think if you have a "live-system" running with importnat data on it, you shouldn't use DSM 5.1 at the moment!
