Problems with transfers when using 4091 SCSI-2 controller or other Zorro-3 boards. Check for revision of Super Buster; the revision 9 chip had problems with Zorro-3 bus arbitration. Revision 11 of the Super Buster fixed this problem. Note that this problem is not due to DMA transfers, but Zorro-3 bus arbitration, so it is possible to see it with any Zorro-3 board.
Problems may be encountered with the A2091 or GVP Series II SCSI controller boards. To isolate this problem, check disk transfers to Chip RAM with a program like Diskspeed 4.2. This problem can be fixed by replacing PAL U209 on the A3640 daughterboard with a revision 3 version.
There can be a software component to these problems, also. Check that libs:68040.library is at least version 37.30. Note that due to the way the library version numbers are handled, version 37.4 is an earlier version. The later 37.30 library correctly handles Zorro-3 transfers and mapping of Zorro-3 boards (for instance, Commodore's SCSI drivers require that copyback caching be disabled during DMA, and this version of the library does that).