I'm tunning my home server. It is a real old goody NEC Express5800, with MSI694D Pro Dual motherboard (VIA chipset and Nvidia VANTA LT AGP card). There is an Adaptec 29160N Ultra160 SCSI card, with three hard disk on it (different time, different speed):

lsscsi
[2:0:2:0] disk QUANTUM ATLAS10K3_18_WLS 020W /dev/sda
[2:0:3:0] disk QUANTUM ATLAS10K2-TY184J DDD6 /dev/sdb
[2:0:4:0] disk MAXTOR ATLAS10K4_73SCA DFV0 /dev/sdc

The fastest is the latest /dev/sdc, I used it as reference!?

hdparm -Tt /dev/sdc
Timing cached reads: 368 MB in 2.01 seconds = 183.30 MB/sec
Timing buffered disk reads: 182 MB in 3.01 seconds = 60.45 MB/sec

step 1.
- attach SATA (hotplug) WD80JD-23JN hdd
using addon card based on SIL3512 chip, as /dev/sdd(1)
hdparm -Tt /dev/sdc
Timing cached reads: 368 MB in 2.00 seconds = 184.67 MB/sec
Timing buffered disk reads: 182 MB in 3.00 seconds = 60.61 MB/sec
hdparm -Tt /dev/sdd
Timing cached reads: 370 MB in 2.01 seconds = 184.07 MB/sec
Timing buffered disk reads: 168 MB in 3.01 seconds = 55.90 MB/sec

step 2.
- attach USB 2.0 (hotplug) SP0411N (SAMSUNG) hdd
using adon card based on VIA6212L chip, as /dev/sde(1)
hdparm -Tt /dev/sdc
Timing cached reads: 212 MB in 2.00 seconds = 105.99 MB/sec
Timing buffered disk reads: 120 MB in 3.01 seconds = 39.88 MB/sec
hdparm -Tt /dev/sdd
Timing cached reads: 212 MB in 2.00 seconds = 105.92 MB/sec
Timing buffered disk reads: 110 MB in 3.01 seconds = 36.33 MB/sec
hdparm -Tt /dev/sde
Timing cached reads: 212 MB in 2.01 seconds = 105.36 MB/sec
Timing buffered disk reads: 72 MB in 3.01 seconds = 23.91 MB/sec

step 3.
- detach USB device
hdparm -Tt /dev/sdc
Timing cached reads: 212 MB in 2.00 seconds = 105.87 MB/sec
Timing buffered disk reads: 120 MB in 3.05 seconds = 35.92 MB/sec
hdparm -Tt /dev/sdd
Timing cached reads: 212 MB in 2.00 seconds = 105.99 MB/sec
Timing buffered disk reads: 108 MB in 3.01 seconds = 35.92 MB/sec
- same as on step 2

step 4.
- detach SATA device
hdparm -Tt /dev/sdc
Timing cached reads: 212 MB in 2.00 seconds = 105.88 MB/sec
Timing buffered disk reads: 124 MB in 3.04 seconds = 40.84 MB/sec
- slightly better then on step 2

How are you interpret these results? Why SCSI subsystem does not go back to previouse state, after plug/unplug an USB device? These "measure" is relevant?

I'm quite confused with these results, could some one figure out a better benchmark test?
My target is to build a stable, resonably fast home server, with many large applications, extensivelly use and share storage resources, such as SAMBA, PostgreSQL, Apache2, PHP and Squirrelmail.