What happens when you cd /mnt/diskonkey/ ? Can you not see your files?
You need to mount /dev/sdc1, not /dev/sdc, and looking at your fstab entry it looks like Kudzu saw the usb drive at boot and added an entry for it.
On my Nikon Camera, also USB storage device, I just mount /dev/sda1 to /mnt/nikon and I can read/write to the device. If you mount /dev/sdc1 and cannot write to it, are you sure there is no write protection on the keychain device?
You should get an entry in your /var/log/syslog similair to:
Jul 24 10:31:15 lightsaber kernel: scsi0 : SCSI emulation for USB Mass Storage devices
Jul 24 10:31:15 lightsaber kernel: Vendor: NIKON Model: NIKON DSC E775 Rev: 1.00
Jul 24 10:31:15 lightsaber kernel: Type: Direct-Access ANSI SCSI revision: 02
Jul 24 10:31:15 lightsaber kernel: Attached scsi removable disk sda at scsi0, channel 0, id 0, lun 0
Jul 24 10:31:15 lightsaber kernel: SCSI device sda: 503808 512-byte hdwr sectors (258 MB)
Jul 24 10:31:15 lightsaber kernel: sda: Write Protect is off
Jul 24 10:31:15 lightsaber kernel:
/dev/scsi/host0/bus0/target0/lun0: p1
Jul 24 10:31:15 lightsaber kernel: WARNING: USB Mass Storage data integrity not assured
Jul 24 10:31:15 lightsaber kernel: USB Mass Storage device found at 4
Jul 24 10:31:15 lightsaber kernel: USB Mass Storage support registered.
Check that there are no errors there. | 
