@darkknigt : no special recommandation but just to let you know, before I upgrade to version 6 my servers were already registered to Synology's Quickconnect service. So I had a valid MAC and S/N generated with the excel spreadsheet. Then when I upgraded to version 6, I just had to replace the PID and VID and everything went ok.
@ oswaldini : i have 2 N40L with 6 HDDs inside with one HDD using eSATA. No issue detected. Everything's OK !
and I just did the "6.0.2-8451 update 8" update with no problem.