[delphin]

In theory, I believe that both slots can use 32 Gig cards at the same time, but some large SD cards have cluster/sector size limits which may keep them from formating correctly.

I have run into an 8Gig card that would not work in my PRS-650 until I reformatted it. Some larger 16 and 32 Gig cards may not work at all or may format to 1/2 or 1/4 size when formatted in a way that is compatible with the Sony.

This incompatibility can happen because some SD cards use low cost flash memory which limits the card to allocating storage in very large chunks, but many USB devices will not recognize storage sectors above a certain size.

I have found that this is more of an issue above 8 Gigs.

On a practical basis, anything above 4 to 8 Gigs is not going to work very well unless you are mostly using that extra space for large audio book files, because if you are instead trying to load several thousand smaller e-book titles, THINGS WILL BECOME VERY VERY SLOOOOOW. The slowdown is mainly for loading new books (which can take several minutes to re-index) and when exiting a book title back to the home screen.

I have a couple of gigabytes of book currently on my reader and that works fine with only minor delays using the standard Sony firmware, but anything more than that and things start to become pretty unwieldy.

You can get around most of these issues by hacking the Sony's software and using PRS+ to disable automatic scanning and indexing for the SD card, and then doing your navigating using folders, but there are trade-offs (you can't search book titles that haven't been indexed from your home screen thumbnail browser for example).