The previous principle (all or nothing) only applies to LVD. For all the transfer protocols below LVD, you can/will run each device at its max-rated transfer rate - as long as the adapter/controller supports that protocol. In other words, As long as your adapter supports the UW (UltraWide) protocol, you can run an UW CDROM (like the Plextor I have) at 40MB/s, and a SCSI-2 (also called fast SCSI) at 10MB/s. Each will operate at their max speed. I had this wrong at one time, thinking that all SCSI protocols defaulted to the lowest common denominator. I think it was Darkheart who showed me the light on that.

If all this SCSI protocol is confusing, don't feel bad. You're not alone. But sooner than you think, it will start making sense to you. Really, you only need to learn about your particular config. You don't have to know everything about SCSI.

Just to confuse you a little more, here's one more protocol nuance: you can run both U2W (LVD, 80MB/s max) & Ultra160 (also LVD, 160MB/s max) each at their own max-rated speeds - as long as the adapter is U160-rated. In other words, if you have an U160-rated controller, and two hard drives - one U2W, and one U160 - the U160 drive will run at U160-rated speeds (160MB/s max), and the U2W drive will run at 80MB/s (U2W speed). 

So it should be getting clear that LVD is the dividing line. At or above it, individual devices won't affect/degrade other devices. While, the same goes for devices below the LVD protocol. The is assuming, naturally, that the SCSI adapter supports the max-rated speed/protocol of the fastest device you want to use. Naturally, you cannot run LVD devices at LVD speeds if your SCSI adapter does not support the LVD protocol. 

Generally, you will have less problems with newer technology. Of course, brand new technology always seems to have its share of growing pains .. but I've found that the older devices tend to have more probs than new ones - in general, with all else being equal. 

Whenever you see the word WIDE, that's talking about 16-bit bus. On the other hand, anything that is not WIDE is NARROW. By definition, NARROW means 8-bit wide bus. To get your data x-fer rate, you multiply bus bit width times frequency (MHz). In other words, 16-bit wide bus (WIDE) times 40MHz (U2W runs at 40MHz) = 80MB/s. Somebody let me know if I screwed up the math. But I'm pretty sure that's right. 

A few years ago, installing a SCSI system in your PC could've been a configuration nightmare, but no more. Since most SCSI cards now have a bios, they install surprisingly easy. For the Tekram DC390-U2W adapter, when you install, let Windows install the generic Symbios driver, then update that driver with the current one from Tekram.

Many SCSI enthusiasts like to point out SCSI's advantages over the IDE/ATA interface/devices .. such as, longer warrantees (typically 5 yrs vs 1-3 for IDE), ability to use more devices (up to 15) with less IRQs. There are other advantages, & some get technical, but all that doesn't mean a hill of beans if you can't notice any improvement in performance. That's where SCSI really shines - in actual use. 

EvilHorace has the Tekram DC-390U3W card and reports that you you'll get a STOP ERROR when trying to install Windows, and that this is solved by hitting the F6 key when prompted, then installing the Tekram drivers you you should have ready & waiting on a floppy (previously downloaded from Tekram site). He says this procedure is described on page 62 of the manual. Thx Evil.

Regarding setting up a Quantum Atlas 10K II on a Tekram DC390-U3W adapter, I read a post where Puppet5 said this: 

You may have trouble recognizing the Quantum's. After your first boot, and before you start to load more drivers as Windows asks, go to the Device Manager. Under SCSI, you'll see 2 entries for you card. Each entry is one channel. If have a yellow ! next to one of them, and you've already loaded the drivers for both channels, and your system is running in DOS compatibility mode (check the Performance tab) click on the offending channel and click the Settings tab. There you will see a blank, white box. In that box type: DisableDomainValidation=1, and reboot. The controller should now recognize your Quantum's as U160 and you system will return to 32bit operation. You may also need to update SCSI card's BIOS, too.

ASPI stands for Advanced SCSI Programming Interface.