The Z has problems with various brands of SD cards. Most will work fine in normal circumstances, but may of them will need to be unmounted / remounted after a suspend. So they will appear to be ok untill you start using them heavy, as in having open applications on them (which prevents sdmgr from unmounting the card). Also noticable if you are using /home on the SD.
I do know that Lexar cards work good, PNY ones need remounting after suspend, and some Sandisk cards have had problems both with heavy app use on the SD, and when formated with the ext2 filesystem. BTW, I just picked up a 256meg Lexar for around $80.00 at walmart, one of the better deals I\'ve run accross, and it works perfectly, same with my older 64meg Lexar card (both /home on SD, and full root filesystem on the SD, ext2 formated).