First open a terminal with Alt-Ctl-T.

Enter the command:

mount

and make a list of which devices are in use.

Enter the following command to keep an eye on the syslog (log of what is happening on your system from the operating system point of view).

tail -f /var/log/syslog

Now plug in and remove the flash drive. Make sure of what device it is, and that it's not one of the ones you saw mounted above. It may be something like /dev/sdc. Also see if you see the same device with a number after it, e.g. /dev/sdc1. If you see that your flash drive is being seen by your system.

Assuming it is not you can format the drive. If /dev/sdm was the drive your flash drive becomes, plug it in once again and enter:

sudo fdisk /dev/sdm

Now you can partition the device and format those partitions as you desire.

If nothing is noted on /var/log/syslog as you plug in the drive and unplug it, it is, indeed, unusable with Ubuntu.