Two important SCSI rules to remember:
When using a Wide SCSI controller (e.g. Adaptec AHA-2940UW), you should be aware of these special issues (from Adaptec's website).
I've read that Ultra SCSI requires active termination. The termination on a Zip drive is probably not adequate for an Ultra SCSI chain. If you're having problems with such a setup, then you could try using another device at the end of the chain to provide termination, or you could buy an active terminator plug to attach to the Zip (try looking here).
If you have an external Jaz drive, I would recommend that you not rely on its Auto (A) termination setting. Set it to On (1) or Off (0) - whichever is appropriate for your configuration. The Auto setting only works when the drive is powered on. And since many SCSI controllers are left to set their termination automatically, I wonder how reliable it would be to have more than one device auto-terminating.
If the SCSI controller has a BIOS, then selecting the right BIOS settings can be important. I don't have experience with a wide variety of SCSI controllers, but I'll list what I think are some of the relevant settings I've chosen with the SCSISelect utility for my Adaptec AHA-2940UW (with BIOS version 1.25).
On the SCSI Device Configuration menu, under the SCSI ID that the Iomega drive is using...Elsewhere in SCSISelect, I've pretty much left things at their default settings (the setting with the * to the left when viewing the list of available settings) although I've set 'Support for Ultra SCSI Speed' to "Enabled" because my HD and my 2GB Jaz drive are Ultra SCSI drives. You can probably find more information on these settings by searching Adaptec's support information.
If you're using a Jaz Jet PCI controller (which I believe is the same controller that is used on the Iomega Buz), then look under SCSI Configuration, and set Synchronous Transfer and BIOS Target Control as recommended above (they are equivalent to Initiate Sync Negotiation and Include in BIOS Scan respectively).
Whatever kind of SCSI controller you have, if it has a BIOS, then make sure it doesn't take control of your Zip or Jaz drive. If it does, then it will probably handle it as a fixed HD, and ejecting/changing disks at the wrong time could result in data loss or even disk corruption. The only time you would want the BIOS controlling a removable drive would be if you're booting from it; in that case, BIOS control is necessary.
Generally, Windows 9x works best when you're not loading any DOS drivers from Config.sys or Autoexec.bat. If you haven't read THIS, then you probably should. If you want those drivers for when you Restart the computer in MS-DOS mode, then there are better ways to load them. I think the best way is to edit the Exit To Dos PIF file in the Windows directory so that it loads everything you need for DOS mode from its Config.sys and Autoexec.bat boxes (as if from a floppy boot disk) which are enabled when you Specify a new MS-DOS configuration. See these Microsoft Knowledge Base articles for more info:
View the Properties/Settings for the Iomega drive in the Disk drives section of Device Manager. Disconnect, Sync data transfer, and Removable should be checked there. No other boxes should be checked.
If you find that Int 13 unit is checked and grayed out, I think that means your drive is under BIOS control; and you'll need to take steps to prevent that. If you find that Int 13 unit is simply checked, then try unchecking it, and reboot the computer. If it is checked again after rebooting, then don't worry about it.
The DOS ASPI manager for a SCSI drive is simply the ASPI manager for the SCSI controller.