They do make 2.5" SCSI hard disks - these are not cheap however
Of course a scsi with 32bit DMA would be prefered - even an 030 should be able to do about 6MB persec with little CPU load!
But as you mentioned sourcing compatible 5v chips would be difficult - combine that with implementing them on the PCB would increase the complexity of the Board - perhaps way to much or even force a 6 layer instead of 4 layer - which in turn can almost double the cost =(
By far a better idea would be too look at an IDE off load engine ? somthing like an intergrated chipset - but again find a 5volt tollerant component - and yet you still increase the complexity several fold - a true pitty
its good to see the bench mark of the 030@28 - of over 6.5MIPs for 28Mhz great stuff indeed!!! I look forward to seeing the 030@50 tests
I am suspecting about 11 MIPs - a clear 2 MIP's (18% faster than a 50MHz Blizzard)