I had posted here (https://mandrivausers.org/index.php?showtopic=43275) a question about a seemingly similar problem.
My system recognized that a usb device was connected - but the external drive was not showing up as "sda" or anything else in diskdrake. (I wanted to reformat to ext3)
The drive was a SimpleTech 250gb usb 2.0. It came formatted as NTFS. It also had "one touch backup software" preinstalled on the drive.
Scarecrow was kind enough to respond to my inquiry, but seemed to be answering a question other than the one I asked. Perhaps I did not phrase my question as clearly as I could have....
After a closer examination of the scant instructions included with the drive I noticed that in order to prep the drive for use with a Mac (reformat to HFS+), you would have to remove the backup software from the drive, format as desired, and then move the software back to the external drive.
This leads me to suspect that the presence of the backup software was somehow preventing my linux system from accessing the drive. I wonder if I had connected the drive to a Mac or Windows machine and removed the backup software.... would my linux box have recognized it?
By the time I came up with this idea I had already arranged to sell the drive to a friend who uses windows - I then bought a drive and and external case, put it together and formatted it, and life is good.
by the by, I had previously used a different SimpleTech external drive on my system with no problems - but that drive had no backup software preinstalled. (It was a 60gb bus-powered drive)
I hope this helps.