Hmm, unless there's a mistake in your mount command this seems to mean that DSL can't read the file system on the USB drive.
Check that the USB drive isn't formatted in exFAT, NTFS, or anything other than FAT32, FAT16, or a Linux file system. In DSL you can use "sudo fdisk -l" to list the partition structure of all the drives. If "HPFS/NTFS" is shown in the "system" section of the listing for the USB drive, it will probably need to be reformatted for use with DSL.
You might want to post the result of "sudo fdisk -l" here.