Stumbled on this today: USB thumb drives that appear to work fine under Windows, but look like they have a bad partition table under Linux, actually have no partition table, the file system is using the whole device. So to use it mount the whole device /dev/sdd instead of a partition number like /dev/sdd1.
I instinctively typed "mount /dev/sdd1 /mnt/foo", and got the message "mount: you must specify the filesystem type". This usually means mount couldn't automatically figure out what type of file system it is and gave up. I tried 'fdisk -l /dev/sdd' to try to figure out the file system type and got back a very broken partition table though fdisk did give a warking "This doesn't look like a partition table. Probably you selected the wrong device."
Perhaps some of the nice graphical desktop stuff already knows this and does the right thing. Since the file system data can be random it can appear to have a partition table, but will be a completely bogus one.
On closer inspection, the kernel/udev, have only created a device for the whole disk since the partition table didn't look valid, so basically Linux mount, instead of saying "file not found", says, "you must specify the filesystem type". Great, Thanks, that was helpful.
Hope This Helps (somebody else)