So /dev/sda doesn't exist? I've seen this before, and rebooting fixed it for me then. Check if it shows up in fdisk first though. Maybe try a different USB port too. Also, resize your partitions from within windows if you're planning on keeping windows.
There is a chance that the partition table is corrupt.
I usually do this: 1. boot and run Live on Linux OS 2. use fdisk, to check that partition table is still OK , 3. if that is OK, then just use fdisk to re-write partition table 4. get back to gparted and it should be showing all partition, in graphic form.
Same problem. I have tried Linux Mint 12 and 13 - the installer does not see any partitions. fdisk, parted and gparted all work. I even deleted my Windows partitions and tried, then tried adding suitable Linux partitions, but the installer cannot see them.