Which distribution, and which desktop environment are you using.
What filesystem does the pendrive use.
If you look at "ls /dev/sd*" before and after inserting the drive you may discover what device the partition is assigned to.
A device node like /dev/sdb is the entire drive (the second one). A device node like /dev/sdb1 is the first partition on the second drive.
You can manually mount the drive, but you need to know the filesystem first. If it wasn't formatted or is corrupt, that could be the problem.
You can get information on a pendrive using the "udevinfo" and "fdisk" programs. Here is an example:
Using "sudo /sbin/fdisk -l" this part is for a pendrive:
Disk /dev/sdc: 256 MB, 256900608 bytes
16 heads, 32 sectors/track, 979 cylinders
Units = cylinders of 512 * 512 = 262144 bytes
Disk identifier: 0x059b7eed
Device Boot Start End Blocks Id System
/dev/sdc1 1 979 250574+ 6 FAT16
udevinfo -q env -n /dev/sdc1
The ID_FS_TYPE line indicates that this pendrive uses the fat file system. So to mount it I would use the filetype "vfat".
Since my pendrive auto mounted, lets look at the mount options that were used:
/dev/sdc1 on /media/CRUZER128 type vfat (rw,nosuid,nodev,noatime,flush,uid=1000,utf8,shortname=lower)
So to manually mount the filesystem on the mount point /mnt/cruzer, I would use:
sudo mount -t vfat /dev/sdc1 /mnt/cruzer -o rw,nosuid,nodev,noatime,uid=jschiwal,utf8,shortname=lower
You would use your own username instead after "uid=". Other options to consider are "fmask=0117,dmask=007". This will allow you to read and write files and enter directories. The "noatime" will extend the life of pendrives by not updating the access time when a file is read.
One quick thing to check if you use the grub boot loader is that you don't use the "noacpi" boot option. This will prevent udev, hal and dbus from running. These services are used by KDE and Gnome for automounting to work. Without them you won't see a popup when you insert a disk.
In kde 3.5, you can right-click on the desktop and select "Configure Desktop". Then click on the "Behavior" icon on the left pane. Then click on the "Device Icons" tab above the right hand pane. Make sure that "Mounted Removable Media" and "Unmounted Removable Media" are both selected. Click on OK. Now hopefully you will see an icon for the pendrive, despite the fact that it isn't mounted. Right-click on the icon and select properties. Click on the Mounting tab. Make sure that "Mount automatically" is selected. If the filesystem is vfat, then uncheck "Access time updates" and select "Flushed IO" , "UTF-8 charset" and "Mount as user". Click OK. If the reason it wasn't automounting is because "Mount automatically" wasn't selected, this may fix the problem.