So it seems DS409slim, faked as DS411j, is a bit too slim for DSM v5. So let’s rollback to the official (nearly) supported version.
First thing to do is to decide which DSM v4 to go for. Looking at the Release Notes, 4.2-3211 is the last common firmware version (between 409slim and 411j). Previous version is 4.2-3202
Log in the DS409slim using SSH.
dharma> cp -p /etc.defaults/VERSION /etc.defaults/VERSION.orig dharma> cp -p /etc/VERSION /etc/VERSION.orig dharma> cat /etc.defaults/VERSION majorversion="4" minorversion="2" buildphase="0" buildnumber="3202" smallfixnumber="0" builddate="2013/03/04" dharma> cp -p /etc.defaults/VERSION /etc/VERSION
Download 4.2-3211 firmware for DS411j (http://usdl.synology.com/download/DSM/4.2/3211/DSM_DS411j_3211.pat).
Now, log-in to the DSM Web interface. From the “Control Panel”, go to “Update and Restore”. Click the “Manual DSM Update”. Select the downloaded firmware and click “OK”. Validate the warning message and wait until downgrade is processed. Let the system reboot and get to the DSM 4.2 log-in screen.
In my case, the password was wrecked. So I had to reset the whole syno to be able to log back-in. Once done, you’ll see that the syno is back to DSM 4. From here, you can simply configure your syno back to DS409slim.
# vi /etc.defaults/synoinfo.conf /etc/synoinfo.conf ... unique="synology_88f6281_409slim"
No data were lost. And 409slim runs fast again. Great!