Manually Mounting a Pendrive :
At first whenever you encounter a Problem with USB devices,first check the latest debug information generated from kernel just after you plugin your device and/or just after you encounter the problem by using cmd as
$lsub //to see list of usb devices
$sudo fdisk -l //to see all attached storage devices and their partition
$sudo mkdir /media/external //creating mount point
$sudo mount -t vfat /dev/sdb1 /media/external -o uid=1000,gid=1000,utf8,dmask=027,fmask=137 //The options following the “-o” allow your user to have ownership of the drive, and the masks allow for extra security for file system permissions. If you don’t use those extra options you may not be able to read and write the drive with your regular username.
or otherwise if the device is formatted with NTFS, run:
sudo mount -t ntfs-3g /dev/sdb1 /media/external
1.Use ‘dmesg’ cmd to get necessary details.
2.Check for User Priviledge:
Go to System->Administration->User and Groups, choose the user, click on “Properties”, then go to the “User Privileges” tab. You should have the “Access external storage devices automatically” option checked.
If your usb device doesn’t appear on your desktop, you should check that the automount action is enabled in the preferences:
Navigate to “System” > “Preferences” > “Removable Drives and Media”
Verify that all “Mount removable drives when…” are checked.