Why would drive run at UDMA33? (Segate 80GB)

Jonathan Arnold jdarnold at buddydog.org
Fri Feb 13 06:58:49 PST 2004

(Still going through some old messages, but this thread had some
misconceptions and myths that I'd like to straighten out):

> ATA channel 0:
>     Master:  ad0 <ST380021A/3.19> ATA/ATAPI rev 5
>     Slave:  acd0 <CD-RW 24X10X40/Y.IW> ATA/ATAPI rev 0
> ATA channel 1:
>     Master:  ad2 <ST380011A/3.06> ATA/ATAPI rev 6
>     Slave:       no device present
> Here is your problem. On channel  0  You have an UDMA100 disk and an
> UDMA33 cd-rw. The motherboard IDE controller steps down the speed to
> the speed of the slowest device. You have to move the cd-rw device

This is not true. With today's computers, all disks will operate at
their highest speed, not matter what other device they are paired with.
Their transfer rate may be slowed down if *both* devices are accessed at
the exact same time, but that's nothing to worry about generally. So just
because you have a CD-ROM and an UDMA100 disk on the same channel,
it doesn't mean the UDMA100 disk will be slowed in nearly any noticable

Also mentioned in this thread was something about the cable being
connected "backwards". There is no "motherboard" and "disk" connecting
direction in an IDE cable, be it a reguler one or a UDMA 100 one. Cables
are made to be a little more convenient if you hook them up the "right"
way (with two connectors closer together at one end), but it has no
bearing on the speed or the UDMA detected.

Jonathan Arnold     (mailto:jdarnold at buddydog.org)
Daemon Dancing in the Dark, a FreeBSD weblog:

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