Okay, first of all, to clear up a little myth:
Remember how higher numbers is supposed to be better (until it came to a panel monitor where the Required resolution resulted in tiny, very tiny print)??.
ATA 44/48 (UDMA3), ATA66 (UDMA4) through SATA 150 and beyond are limited to hard drive mechanical speeds. Physical hard drive speeds rarely exceed 48 megabytes per second sustained read or write speed. ATA, SATA do not read and write at the same time without going to half speed or less. A good DVD burner is unlikely to exceed ATA33 specs (A better DVD burner will use UDMA33 or UDMA66 on 80 wire cable to increase "burn-proof" performance).
The faster speed does do something. The tiny, nearly insignificant memory buffer on the hard drive can communicate with the mainboard at the rated speed. That is, until the buffer is empty. In the blink of an eye, its buffer is empty and usefullness is done.
The ata100, 133, 150 speeds are not beneficial for single drive use. They are for a RAID implementation whereby two identical drives are hooked up on an 80 wire cable with both drives set on Cable Select. By this, you can achieve double speeds. The secondary drive controller can then be equipped with an identical configuration for redundancy. Yes, ordinary IDE does have built-in raid capabilities in hardware. Otherwise, speeds beyond ATA 66 are useless and actually slow down sustained write speeds on many current hard drives.
An 80 wire ribbon is a requirement for all UDMA, but ordinary ata33 will run on a 40 wire ribbon, so there is some confusion between the Sony optical Ata33 (requires 40 wire) and the BenQ optical UDMA33 (requires 80 wire). This is only a problem with the "33" speeds.
Slaving drives should be done in decending order if the drives do not match and cannot be "set" with a utility.
An ATA48 and UP drive cannot share a cable with a drive that is not meant for an 80 wire cable. Change your hardware.
A UDMA33 drive cannot share a cable with a drive that is not meant for an 80 wire cable. Change your hardware.
An ATA-33 drive can share a 40 wire cable with any other ATA-33 and lower drive.
After reconfiguring drives, you need to clear your CMOS if your computer is based on the Via chipsets. However, if you have all drives/only drives that are ATA48 and UP, you do not have to worry about clearning the CMOS.
The VIA bug was never worked out. It is still a harsh reality for any ATA33 and lower drive that is plugged into a Via chipset. Reseting the CMOS and then leaving the IDE channels on Auto Detect is a partial cure. The VIA bug is incompatible with "Cable Select" feature, so use the master and slave jumpers where appropriate. Zero jumpers may also be necessary for "master" depending on brand of device. *For best results, avoid ata33 and lower drives when using VIA based motherboards (Use only UDMA33 and UP drives so you can avoid the Via Bug). *If your ATA 133 drive is paired [on same cable] with one that is not ATA133, reset it to the same communication protocol as the other drive on the same cable (using the maufacturer's provided utilities) as this will result in much faster performance.
*If you have a seemingly permanent change after flashing a drive, it could be that the motherboard is "stuck" on the wrong communication protocol. This also happens when removing an old optical drive to install a new optical drive. In both cases, clear the CMOS and leave the IDE channels in use to Auto Detect (you may "set" hard drives if they sporadically show up the wrong sizes).