It is a very strange problem. My usb harddisk has four partitions, one is primary, the other three are logical (contained within an extended partition). When I plug in the disk, three of the partitions are mounted automatically except one--the first logical partition in the extended partition. Initially I thought it is the problem of system (at that time I used Mint). But after I change to Ubuntu 12.04, the problem wasn't solved. I don't want to add a rule in fstab, and I want to know what happened. The disk is fine, and the partition can be accessed in Windows and mounted manually. result of dmesg | tail: [100933.557649] usb 2-1.2: new high-speed USB device number 4 using ehci_hcd [100933.651891] scsi8 : usb-storage 2-1.2:1.0 [100934.649047] scsi 8:0:0:0: Direct-Access SAMSUNG PQ: 0 ANSI: 0 [100934.650963] sd 8:0:0:0: Attached scsi generic sg2 type 0 [100934.651342] sd 8:0:0:0: [sdb] 625142448 512-byte logical blocks: (320 GB/298 GiB) [100934.651977] sd 8:0:0:0: [sdb] Write Protect is off [100934.651989] sd 8:0:0:0: [sdb] Mode Sense: 03 00 00 00 [100934.652836] sd 8:0:0:0: [sdb] No Caching mode page present [100934.652848] sd 8:0:0:0: [sdb] Assuming drive cache: write through [100934.655354] sd 8:0:0:0: [sdb] No Caching mode page present [100934.655367] sd 8:0:0:0: [sdb] Assuming drive cache: write through [100934.734652] sdb: sdb1 sdb3 < sdb5 sdb6 sdb7 > [100934.737706] sd 8:0:0:0: [sdb] No Caching mode page present [100934.737725] sd 8:0:0:0: [sdb] Assuming drive cache: write through [100934.737731] sd 8:0:0:0: [sdb] Attached SCSI disk result of parted -l: Model: SAMSUNG (scsi) Disk /dev/sdb: 320GB Sector size (logical/physical): 512B/512B Partition Table: msdos Number Start End Size Type File system Flags 1 32.3kB 21.5GB 21.5GB primary ntfs 3 21.5GB 320GB 299GB extended lba 5 21.5GB 129GB 107GB logical ntfs 6 129GB 236GB 107GB logical ntfs 7 236GB 320GB 83.8GB logical ntfs