Okay, I think I'm on to something.
Tried a virtual machine (again, host is macOS Sierra). Tried using both Ubuntu and XP (not at the same time, of course ) as virtual machines to copy ROMs over. Result: didn't help. It looks like if Sierra is involved in any way, it doesn't work.
Now, here's what I discovered when futzing with Disk Utility:
1) It shows up as a 265k device...which is odd because it should be, what, 144k when I have the switch to 144? Anyway, this is nothing new -- it did that before I upgraded.
2) It shows that 134k are used -- which is odd because, well...there's nothing on it. It does correctly do the 265-134 math and show 132k free, though.
3) The big thing: it's showing up as a FAT12 volume.
I think #3 is the key here. Is this indeed FAT12?? If so, that might be the problem. Do a Google search for osx fat12 or macos sierra fat12, and...well, not much there. I'm wondering if Sierra knows how to handle that. I think in terms of MS, Mac can only natively read and write to FAT and FAT32, and with the proper 3rd-party drive, NTFS. So that it's FAT12 might be throwing things off.
Juan - have you made any progress since you upgraded to Sierra?