The problem corresponds to the well-known 137GB barrier.
And the problem with the Seagate 7200.9 HD comes from their buggy firmware from what I understood. However, concerning your RAID array, there are so many other factors that might play a role. But true, the 7200.9 are also slow at reading, slower than for writing actually!
Do you have a source for that?
This seems to contradict that:
http://www.storagereview.com/php/benchmark/bench_sort.php
Some recent Seagate drives have had severe problems especially some 7200.8 models.
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Rank Drive Name Model Results in MB/Sec
1 Maxtor Atlas 15K II (147 GB Ultra320 SCSI) 8E147L0 97.4
2 Fujitsu MAU (147 GB Ultra320 SCSI) MAU3147NP 93.8
3 Seagate Cheetah 15K.4 Server Mode (147 GB SAS) ST3146854SS 93.4
4 Hitachi Ultrastar 15K147 (147 GB Ultra320 SCSI) HUS151414VL3600 93.2
5 Seagate Cheetah 15K.4 Server Mode (147 GB Ultra320 SCSI) ST3146754LW 93.2
6 Seagate Cheetah 15K.4 Desktop Mode (147 GB Ultra320 SCSI) ST3146754LW 93.2
7 Maxtor Atlas 10K V (300 GB Ultra320 SCSI) 8D300L0 89.5
8 Fujitsu MAT (300 GB Ultra320 SCSI) MAT3300NP 88.5
9 Western Digital Raptor WD1500ADFD with NCQ (150 GB SATA) WD1500ADFD-00NLR0 88.3
10 Western Digital Raptor WD1500ADFD no NCQ (150 GB SATA) WD1500ADFD-00NLR0 88.3
11 Western Digital Raptor X WD1500AHFD with NCQ (150 GB SATA) WD1500AHFD-00RAR0 87.3
12 Western Digital Raptor X WD1500AHFD no NCQ (150 GB SATA) WD1500AHFD-00RAR0 87.3
13 Hitachi Ultrastar 10K300 (300 GB Ultra320 SCSI) HUS103030FL3600 86.3
14 Seagate Cheetah 10K.7 Desktop Mode (300 GB Ultra320 SCSI) ST3300007LW 76.5
15 Seagate Cheetah 10K.7 Server Mode (300 GB Ultra320 SCSI) ST3300007LW 76.5
16 Seagate Barracuda 7200.9 no NCQ (160 GB SATA) ST3160812AS 72.7
17 Seagate Barracuda 7200.9 with NCQ (160 GB SATA) ST3160812AS 72.7
18 Seagate NL35 no NCQ (400 GB SATA) ST3400832NS 71.9
19 Seagate NL35 with NCQ (400 GB SATA) ST3400832NS 71.9
20 Western Digital Raptor WD740GD-00FLA1 with TCQ (74 GB SATA) WD740GD 71.8
21 Western Digital Raptor WD740GD-00FLA1 no TCQ (74 GB SATA) WD740GD 71.8
22 Western Digital Raptor WD740GD-00FLC0 no TCQ (74 GB SATA) WD740GD 71.8
23 Western Digital Raptor WD740GD-00FLC0 with TCQ (74 GB SATA) WD740GD 71.8
24 Samsung SpinPoint P120 no NCQ (250 GB SATA) SP2504C 71.3
25 Samsung SpinPoint P120 with NCQ (250 GB SATA) SP2504C 71.3
26 Seagate Barracuda 7200.8 no NCQ (400 GB SATA) ST3400832AS 69.8
27 Seagate Barracuda 7200.8 with NCQ (400 GB SATA) ST3400832AS 69.8
28 Seagate Barracuda 7200.9 no NCQ (250 GB SATA) ST3250824AS 67.8
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