Wow, that is an excellent fast response! You guys are the best.
The files are converting quite happily now.
However, when I try to boot the Mirage, it is now reporting "UD" (unformatted disk) for all the disk images I try to boot with. The RA signal comes on the HxC when the Mirage tries to read the disk, but the track/sector readout doesn't advance. "UD" is what you would expect to see with a bad disk, or certain disks that don't contain the OS for the Mirage's initial startup sequence, but this is also more or less what the Mirage was reporting with its original disk drive, so now I can't be sure whether there is a problem with the files, or an electronic fault with the Mirage's disk controller.
I really need to get in touch with somebody else with a Mirage/HxC combo so I can troubleshoot using a confirmed working MASOS boot image. I hope my keyboard isn't fried!
(firmware version is 220.127.116.11 ; jumper setting is bottom right (Drive A=ID0), ribbon is the original Mirage ribbon, pin1 on the right - these are the same settings as I saw in Youtube videos of the Mirage working with the HxC. I also tried swapping a different ribbon connector, sadly no luck there - also it seems that if things aren't connected properly the Mirage does not attempt to read the disk and does not report "UD").