Write error issues: make sure the SCSI bus is terminated correctly. You need exactly two terminators, one at each end of the bus. They may sit on the cable ends or within the last devices in the chain.
When only one connector of the 2091 is used, the resistor array MUST be present. If both, internal and external, connectors are used the resistor packs MUST NOT be present. If the resistors are lost you can use an external or cable terminator on the port not being used.
Not terminating the bus properly is asking for trouble. It may work for a while (with a very short bus) but will eventually fail.
1 MB chipmem: nearly all A2k revs (4.1+) have jumpers for 1 MB chipmem settings, no need to solder or cut anything.
Zorro slots: It's a good idea to put the SCSI controller in the rightmost slot, but it show work just about anywhere. If the other slots don't work it's either due to corrosion/bad contact in the slot or a bad _CONFIG chain logic (74LS32 in U606 or RP607).
