[id10t]

Speed and cost. Remember also that on the standard ide interface the 2 devices will go as fast as the slowest one. Also, drive spin speed (rpm) will have an effect on performance.

From 3 linux boxes...

Dell w/ p4 and SATA drive:
hdparm -Tt /dev/sda

/dev/sda:
Timing cached reads: 2924 MB in 2.00 seconds = 1461.49 MB/sec
Timing buffered disk reads: 142 MB in 3.02 seconds = 46.96 MB/sec

Dell w/ P3-500 and ATA100 and 5400 rpm drive:
/dev/hda:
Timing buffer-cache reads: 128 MB in 1.04 seconds =123.08 MB/sec
Timing buffered disk reads: 64 MB in 2.32 seconds = 27.59 MB/sec

iMac-DVD running Debian and a 7200 rpm drive (ATA100)
Timing cached reads: 404 MB in 2.01 seconds = 201.00 MB/sec
Timing buffered disk reads: 40 MB in 3.09 seconds = 12.94 MB/se