When you're trying out an SD card, use a plain vanilla, single partition FAT32 SD card.
After you've seen that it works, convert it to whatever filesystem you like/need.
You have to change the mount. The mount is usually done by fstab.
If you have a root console you can do it manually (but not practical for regular use).
Code:
mount -t vfat /dev/block/mmcblk1p1 /mnt/media_rw/extsd <-- one possible command for Android based things
mount -t vfat /dev/mmcblk1p1 ~somewhere~ <-- one possible command for Linux based things
Obviously that vfat needs to be replaced by ext2 or whatever.
You also have to figure out where you want it mounted.
You might even have to create that directory.
(Sorry, I'm not familiar with PocketBook or where its mount point are.)