it's right, the usbkey has only 1 partition (normaly) but /dev/sda refers to the entire disk (key). So you have to specify the number of the (unique) partition.
It's odd that the mount command hangs the shell without any output.
Are you sure that the right type of usb-module is loaded (ohci/uhci) in other words do other usb devices work?
Is usb-storage and sd_mod loaded?
And what says dmesg after plugging the usbkey?
The output should look like:
usb 2-1: new full speed USB device using address 3
scsi1 : SCSI emulation for USB Mass Storage devices
Vendor: YAKUMO Model: Quicksave 2.0 Rev: 2.00
Type: Direct-Access ANSI SCSI revision: 02
SCSI device sda: 256000 512-byte hdwr sectors (131 MB)
sda: assuming Write Enabled
sda: assuming drive cache: write through
sda: sda1 sda2
Attached scsi removable disk sda at scsi1, channel 0, id 0, lun 0
USB Mass Storage device found at 3