I have two MB6582 and one sammichSID, all with these problems. I used to be able to upload hex files to each of these synths successfully. The only thing that's changed is I got a new computer (Mac OS 10.6.4, before was 10.4.9). My MIDI interface is an M-Audio Firewire 410.
EDIT: I just hooked up my old computer, and was able to upload a hex to the Sammich.
In MIOS Studio, sometimes I get the message:
No response from MIOS8 or MIOS32 core!
Check MIDI IN/OUT connections
and Device I