This post by Mike Calligaro on the Windows Mobile blog points out that most cards and card readers available (at least on Jan 1st, 2007) follow the SD Spec 1.1, which maxes out at 2 gigabytes. Here's an excerpt,
"Up to 2G, the only negative to larger storage cards is that they cost more. They don't burn any more power than smaller ones and they're not inherently any slower than smaller ones. I've used 2G cards in my Windows Mobile devices, and they work fine. Unfortunately, 4G and larger cards aren't likely to work yet.
Most cards and card readers out there follow the 1.1 version of the SD specification. That spec maxed out with 2G cards. There's a new 2.0 version of the spec that allows for larger cards, but Windows Mobile currently doesn't support SD 2.0. Even though we don't explicitly support SD 2.0, it's possible for our OEMs to add support themselves (I don't know if any have). So if you've got a Windows Mobile device that's able to read 4G SD cards, you don't have to call me a liar. Ignorant, yes. Liar, no."
Riki commented on this blog entry, however, and said:
"The maximum capacity of SD card is 2GB under SD 1.1 of SD spec; however, by using FAT32, many SD 1.1 host devices can use 4GB SD cards. SD 1.0/1.1 uses traditional “Byte Addressing” scheme which limits the maximum capacity to 4GB while SD 2.0 adopts “Sector Addressing” scheme."
So it looks like when purchasing new SD memory for a WM5 device, you should go with 2 gigabyte cards--those are guaranteed to work. However, some devices may support 4 gigabytes and up, especially if the OEM implemented the SD 2.0 Spec.
On a related note, my advisor found two programs that turn a WM5 device into a USB SD reader. I haven't used either of these, so I cannot comment on them:
http://www.softick.com/pocket-pc/cardexport2/
http://www.modaco.com/WM5torage-t238545.html
