I get the SD card mounted read-only occasionally if I remove it while the Z is powered on, or remove the card without software ejecting it.
To fix it I software eject the card, put the Z into suspend mode then power on again - this almost always remounts the SD read-write.
If not try rebooting, and if that don\'t work try fsck (or check for errors in windows using a card reader), if that still don\'t work then you\'ll need to try reformatting.
Not I only have a C760 so don\'t know about the 5x00 models
Stu