[Linux][USB] How to mount a USB flash disk ?

If you are not sure what device your system is using, type:
tail -f /var/log/messages

Now insert the usb device.
You should see similar output appear on your console:
Initializing USB Mass Storage driver...
usb.c: registered new driver usb-storage
scsi3 : SCSI emulation for USB Mass Storage devices
Vendor: Generic Model: STORAGE DEVICE Rev: 1.02
Type: Direct-Access ANSI SCSI revision: 02
Attached scsi removable disk sda at scsi3, channel 0, id 0, lun 0
SCSI device sda: 512000 512-byte hdwr sectors (262 MB)
sda: Write Protect is off
sda: sda1
WARNING: USB Mass Storage data integrity not assured
USB Mass Storage device found at 2
USB Mass Storage support registered.
usb.c: USB disconnect on device 00:1d.7-6 address 2

Make note of the line:
sda: sda1
This is the device your system is using.

If it doesn't work then try this: 

mount -t vfat /dev/sda1 /mnt/usb 

If it still doesn't work try this (some usb keys don't have a partition table on them): 

mount -t vfat /dev/sda /mnt/usb

If this still doesn't work, try to reload the modules:

modprobe -r usb-storage 

modprobe usb-storage

Then try to mount again.