When using the Daystar cards and the non-stock ROM, I lost the ability to read/write to 1.4MB floppies even though 400k and 800k disks worked fine. But when using the DiiMO in conjunction with a non-stock ROM, the floppy drive worked fine. Hence, there is an incompatibility with Daystar 040 cards and non-stock ROMs.

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I bet that the problem is in the firmware on the Daystar card. At a guess there's a table in there somewhere which adjusts some timing parameter related to floppy drive operations, according to which model Macintosh the upgrade is installed in. An SE/30 has a 16 MHz bus speed while a IIsi has a 20 MHz bus speed and the way Apple tended to tie the timing of floppy drive operation into stuff on the motherboard, I think it would make sense. But I'm writing off the cuff here.

However, given that the first thing to go in clock chipping experiments is floppy drive operation, it seems like this issue would also be a timing issue and the change in ROM evidence indicates it is something the Turbo 040 changes when it thinks it is in a different machine.

So, who's going to edit the ROMs to make this work? Either edit the IIsi code to report itself as an SE/30, or edit the Turbo040 firmware to use the same floppy timing on a IIsi as it would on an SE/30...