While I have toyed around with HAL very little, I believe that looks like an error caused by HAL, the hardware abstraction layer daemon, which is *supposed* to make auto-mounting such drives easy. Evidently it's not working right for you.
Until somebody else who has a good grip on HAL, comes in to offer help, I will offer just one suggestion, in case it helps meanwhile (assuming that the error is accompanied by a NON-mounted drive):
If there's no entry in your /etc/fstab file for that drive, I suggest adding one. It might cause the thing to be mounted properly/successfully. For help on this, if you decide to try it, examine the man pages for "mount" and "fstab" and read the options you want or need for the filesystem on the drive, and follow the example(s) of the existing entries in your /etc/fstab file.
This may not help, but without knowing for sure if it's a HAL problem (which requires HAL file adjustments) it's the best I can offer at this time.
If you run into troubles with the fstab file, let us know. Meanwhile, I hope somebody pops in sooner than later, who can make another, maybe better suggestion.