Hmm this box should do about 200MB/s.
I hope your Seagate HDD is not a new series STxxxxDM001.
The last time we used 2TB STXXXDM001 in RM5_R5 a five bay 1U rack mount hardware raid.
The results is very funky in write
It is very fast at first, but then it slows down to a craw.
I tried all to force the HDD ran as 1.5/3.0... It does not help.
But the I switch the HDD to Hitachi we got 245MB/sec
I think it's the HDD, try other brand of HDD and see your result is same as mine