I have a DS409 + NAS, and I've been happy with this for the last 3 years.
Recently one of my HDD has been broken. The units where 4 discs WD20EARS. I replaced the failed unit with a WD20EARX with the same size.
I repaired the unit without any problems although the process has taken about 24 hours.
The fact is that from that moment, I noticed the NAS very VERY slow, the NAS is only capable of read speeds above 9-10MBs and before the hard drive broke was capable of speeds around 80-90MBs.
The unit was established with SHR (RAID-5) and has the latest firmware DSM 4.0-2228, all my LAN is GIGABIT with jumbo frames 9k.
Can anyone help me please?
Thanks in advance.