I know this is a little off-topic, but...
I've had this behavior now with three SD cards, a 512MB, a 1GB, and a
2GB. I've been suspecting the Mac OS version, the Mac hardware, and
the adaptor I use to connect the SD card to the USB port, but nothing
correlates.
When I buy a new SD card and try to copy music onto it (typically a
bunch of 4 to 5 MB MP3 files) I start getting a "disk full" error at
exactly half capacity. For a 1GB SD, for example, it starts saying
"disk full" at 512MB. Now, I use the "erase disk" function on the
Macintosh, which I think is probably like a format, and suddenly I can
use the full amount of memory on the card. This happens
consistently. I did at one point look at the partition table on the
SD card and there was nothing particularly strange about it.
Does anybody know enough about SD cards to comment on what about the
initial formatting as done by the manufacturer might be causing this?
-zben