Here's one interesting excerpt from FWB FAQ re. Nubus Jackhammer that I saved many moons ago:

Can I use an array volume as my startup volume?

(the JackHammer makes it appear to the system that SCSI Manager 4.3 is in ROM)

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I'd like to know how they did that. This would be a very convenient (probably essential) component in an IDE card for the old Macs. Also probably necessary for a USB card and anything else that can add storage. A fellow dropped in who provided some details in a thread somewhere around here many months ago, but I only understood about one concept in two of his explanation.

Note
If you have SCSI Manager 4.3 only by way of a Hammer Storage NuBus SCSI JackHammer card software, you can't array devices on the JackHammer bus with devices on buses without SCSI Manager 4.3 (in this case, for example, devices on the native bus).

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But if you do have SCSI Manager 4.3 in ROM you can array drives on both built-in busses and on the JackHammer card. I had a four drive array with two Fast & Wide drives on the JackHammer, one narrow drive on the built-in internal SCSI bus and one narrow drive on the built-in external/internal bus. This was after I'd done testing and found that a third drive on the JackHammer didn't really add any performance, but the narrow drives on the built-in busses did. However, only one drive per bus added performance. A second drive on either of the busses or two drive on only one bus did not improve performance. That was in an 8100/120 clone. (PCC Power 120.)

The drives were all the same model (ST32550) just different interfaces (wide vs. narrow; ST32550W vs. ST32550N).