OK, this is how my Ami now sits:
A4000T (SoftWare Hut) with 400W AT replacement PS (just in case I needed the extra 150W), CS-PPC (SN DC50274, rev. 4 ) 233MHz 604e/ 50MHz 060 + 128M 60ns ram, cybppc.device for SCSI which has Quantum Atlas V 18.2GB (unit 0), White IDE-DVD-R/W 22X (NewEgg) on ACARD SCSI->IDE Unit 1 (A4000T.com), SCSI Multicard reader (A4000T.com ~ mech) - unit 3, ACard SCSI->IDE-> PC card slot (unit 4) -- non-hot swappable. There is the standard A4000 HD floppy drive and a non-standard CPU fan attached to the edge of power supply to blow air across the CS-PPC. THe IDE-to-PC card adapter is attached in back above the power supply with a small slot cut in the metal case. A powered small 4-port USB expander is mounted on top of the case and attached to the Deneb. Off the parallel port is attached an LPT2USB adapter (made in Germany, sold out of AZ in USA). With a fair amount of tweaking the USB ports can drive the 2 HP printers; with a huge amount of effort both printers can be used over my network in OS 4.1 only.
A4000T Slots: De-interlacer card in video slot (waiting for Indivision AGA 1200 Mk2), ZorRam 128M card (AmigaKit), and Deneb USB card (AmigaKit).
Mediator Slots: Mediator card, SB128 card (with internal hardwired Paula-out to AUX-in), Radeon 9200 256M video card (Elbox), "Spider" card (for use in OS 3.9), RTL8029AS card, and finally Generic Sii3114 SATA card (Amaz--.com) with 82 Gb SATA HDD (only for use under OS 4.1).
Available to substitute USB trinkets: USB wireless mouse, Turtle Beach "Amigo II" sound dongle, a plug-in 7-slot card reader (Amaz--.com), and a Trendnet TU2-ET100 ethernet dongle (NewEgg).
Believe it or not the system is relatively stable under both OS 3.9 and 4.1.
OS 4.1 can't 1) Use the "Spider" card, and 2) Can only read from cards in the SCSI card device. It can write to the USB card reader. OS 3.9 can use all of the above, but not the sii3114 SATA card and drive. I have an ESS solo 1 card (1938s) which, if it works, it is not supported in OS 3.9. The SCSI Card reader allows booting from any card under OS 3.9, and I use a small CF card with a high boot priority to boot into OS 4.1. Both OS's can boot from the IDE->CF card off the SCSI chain. I will, on the next gutting, switch out the Acard adapter on the DVD and use the internal A4000's. No, I can not load up the Deneb flash to allow USB booting, the system HATES this. And will not boot to OS 4.1 in any capacity or form (and has "issues" under 3.9); there may be an odd "mad bit" in there among the bytes and megabytes, but I can't score another USB solution at present; I prefer one that works as is for now.
I still love the SCSI Card reader; I can boot from it (in both OS's), back up to multiple cards without rebooting, boot from backups, and copy from CF card to CF card using a PCMCIA adapter. USB is great. The Deneb & Poseidon are amazing products, and in 2012 one absolutely needs this, yes even on a 20-year old piece of hardware (that is still being used). As an aside, how many people use 20-year old PC's and Mac's for productivity?