Test made with 'creative zen' with ID 041e:4157 - firmware 1.21.03e and old debian linux
The better way to use this music player is install Rockbox firmware. With this firmware you can read all music formats, use player like mass storage and have much features.
In zen with original firmware sometimes the player stay on freeze status without disconnect itself to pc even I disconnect it.
mtp-detect command from mpt-tools can unblock it sometimes.
If you get sometimes like “memory is not formatted correctly” you can try to make a deep format:
dd if=/dev/zero of=/dev/sda bs=1M or shred -vfz -n 100 /dev/sda
where sda is your sd-card device