8jschmidt, the file system is FAT and can be read by OSX and Vista with no trouble. However I have seen trouble with XP SP2 where I will get the blue screen of death when the card is read. I have not attempted on SP3 but I know the UC520 will not read the Card Flash if it is formatted on a device other than the UC520 or perhaps another ISR.
Hope that helps,
Douglas Smith
Customer Support Engineer
Small Business Systems Business Unit