sudo lsusb will tell you what USB devices Linux detects. Whether a USB storage device mounts, or is detected, are separate issues.
sudo lsusb -v will give verbose output, possibly more information than you want if the OS truly doesn't recognize the device.
Alternatively, you could compare the lists of devices in
/dev before and after plugging in the USB device. There are many ways to do it; I would probably just use:
ls -l /dev/* | wc -l
This will give you a number of recognized devices. Doing it before and after plugging in a device will tell you if the OS assigned the device in
Another option would be to look at what is happening in
dmesg when you plug in the USB device.
dmesg may tell you things like how a device failed.
If the USB device you are having trouble mounting, is on the lsusb list, then you can try mounting the device. At this point it would be good to know the filesystem type.
sudo fdisk -l will tell you the filesystem type, in the form of an ID. You may have to look up the ID number. There are lots of references online for that. Once you know the device listing, that is,
/dev/hda1 and the filesystem type you can try to mount the device manualy with the
sudo mount /dev/hda1 /home/user/Desktop/whereEver
You may have to make sure the location you want to mount the device on exists. If the OS recognizes the file system, then
mount might just work if the file system is not a native file system type; you may have to specify flags for mounting.
Post back your output from
dmesg (not all of it, only from around when the USB device is plugged in), and
You may find Linux / UNIX: Device files helpful if trying to determine device type.
I am writing this assuming all your unrecognized devices are block type devices.
There are many ways to approach this type of problem and many possible solutions. More specific information is needed to provide a solution.
There are also many GUI applications that can do the same thing. You might try looking for the plugged-in hardware in the "Disk Utility".