It sounds to me to be a permissions issue. I've come across this myself with my Linux Mint 14 (Cinnamon) install, which I believe is based on Ubuntu 12.10.
First thing to do is to check the permissions on the hardware device mounting your card. You'll need to find the device name. To do this run the following command:
sudo fdisk -l
Your device will probably be something along the lines of /dev/sdb1, in my case the SD Reader was /dev/mmcblk0p1.
Next you need to get the permissions on this device:
sudo ls -l /dev/mmcblk0p1
Replace "/dev/mmcblk0p1" with your device location.
The output will look something like this:
brw-rw---- 1 root disk 179, 1 Feb 3 21:58 /dev/mmcblk0p1
This tells us the device is owned by User 'root' and group 'disk'
You need to be a member of group 'disk' to be able to write to the SD card.
You can check which groups your a member of with
In my case I was not a member of the 'disk' group, I rectified this with
sudo usermod -G --append disk username
This adds the group 'disk' to your user's groups, which should allow you to now read & write to the SD card