Today i did a couple of tests on all harddrives of my NAS. There's one disk that kept giving errors in the seagate tools. It's now on its way to Seagate. Hopefully things will be handled quickly so i can go further with my tests. But so far it looks like the problem is discovered.
Will update when i receive a new HD.
Got my disk replaced by Seagate. Yesterday i rebuilt the array, after 24hrs it was done. System works perfectly again.
The SS4200E isn't that good in reporting a broken harddisk, even after doing a system test and dumping it to a log it showed no errors.
Case closed, HD was broken.