Did you find any code in there that made you think so? Generally, the mmc
layer doesn't do much about the layout of the cards but merely represents
what is there.
On the contrary, the USB card readers go through a lot of effort to
make the card look like a SCSI device, and many of these implementations
are buggy in some way. SD cards are defined to have 512 byte sectors, while
USB mass storage can theoretically have a different sector size. Of course
any other size than 512 bytes is likely to break some code, which you have
experienced.
Note that Linux has another driver for USB mass storage, the "ub" block
driver which does not go through the SCSI software layer. It would be good
if you could check whether that has the same problem. Also, it might be
interesting to see if another operating system has the same problem
with the combination of your card and reader.
If it works on another OS, it's probably fixable on Linux, but the
effort may be significant.
The camera is just another implementation of a USB mass storage device on
top of an SD card. My guess remains that this one works correctly, while
your standalone reader has a quirk that prevents it from being used on Linux.
Arnd <><
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