I have a 1.5TB WD external USB HDD and two Ubuntu systems, both 'precise'. When I plug the disk on system A it doesn't show up on the output of sudo fdisk -l nor is it automatically mounted (note that that wasn't always the case - it used to appear in the past). When I plug it on system B (again Ubuntu precise) it shows up when I do a sudo fdisk -l (output appended at the end) and mounts automatically just fine.

What does this discrepancy point to and what kind of diagnostics should I run / tools I should use to troubleshoot the problem?

I followed the suggestion I received to do a sudo tail -f /var/log/syslog and the output is the following when I plug, unplug and replug the USB cable on System A:

Sep 14 23:27:09 thorin mtp-probe: checking bus 2, device 3: "/sys/devices/pci0000:00/0000:00:13.2/usb2/2-2"
Sep 14 23:27:09 thorin mtp-probe: bus: 2, device: 3 was not an MTP device
Sep 14 23:28:01 thorin kernel: [  338.994295] usb 2-2: USB disconnect, device number 3
Sep 14 23:28:04 thorin kernel: [  341.808139] usb 2-2: new high-speed USB device number 4 using ehci_hcd
Sep 14 23:28:04 thorin mtp-probe: checking bus 2, device 4: "/sys/devices/pci0000:00/0000:00:13.2/usb2/2-2"
Sep 14 23:28:04 thorin mtp-probe: bus: 2, device: 4 was not an MTP device
Sep 14 23:29:54 thorin AptDaemon: INFO: Quitting due to inactivity
Sep 14 23:29:54 thorin AptDaemon: INFO: Quitting was requested

(I guess the last two message are irrelevant).

Output of sudo fdisk -l on System B

$ sudo fdisk -l

Disk /dev/sda: 1500.3 GB, 1500301910016 bytes
255 heads, 63 sectors/track, 182401 cylinders, total 2930277168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00070db4

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048  2905114623  1452556288   83  Linux
/dev/sda2      2905116670  2930276351    12579841    5  Extended
/dev/sda5      2905116672  2930276351    12579840   82  Linux swap / Solaris

Disk /dev/sdb: 250.1 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders, total 488397168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x9c849c84

  Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *          63   488375999   244187968+   7  HPFS/NTFS/exFAT

Disk /dev/sdg: 1500.3 GB, 1500299395072 bytes
255 heads, 63 sectors/track, 182401 cylinders, total 2930272256 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0003e17f

    Device Boot      Start         End      Blocks   Id  System
/dev/sdg1            2048  2930272255  1465135104    7  HPFS/NTFS/exFAT