When I got my D2G the first thing I did was go out and buy a 32GB SD card since that is the largest it supports.
To transfer from one card to the other (since I found in the past that a card formatted by a PC may not agree with the phone and vice-versa), I hooked up my old phone and old 16GB card to the computer in one USB port in mass storage mode, and then hooked up my new phone and new 32GB card in another USB port in mass storage, then just dragged and dropped everything from one to the other.
I experienced really weird behaviour during this process. The 15GB transfer took over 4 hours, and then certain folders that should have contained 3GB of files on the new phone, were just showing as empty. I then re-filled them, only to find that another folder had either gone empty or dropped a few files. I kept this up until it would "take" all my files (of course, at this point I was pretty sure I didn't want to keep this card). Finally it appeared as though this card had all my data on it, so I unhooked the USB, tried to mount it, and I was told the card was damaged. Anytime I try to mount that card, I get the same message.
Got a different 32GB card now, and I am getting the exact same thing as above, except that I didn't have the "empty folders, files disappearing" problem I had before.
Is there any kind of a known issue with this phone that causes it to dislike certain SD cards? After one instance it is really easy to conclude the card was faulty. After two instances, I start to wonder about my phone.