Application freeze escalates until whole system locks up

Twice today my machine has locked up in the same way and in the same circumstances. In both cases it started with Firefox freezing. At this point I could still switch between windows move windows around the desktop. However when I clicked the X in the corner to close the Firefox window, the desktop became completely unresponsive. I could still move the mouse pointer around, but I couldn't click on anything.

I could also switch to another virtual console using ctrl-alt-F1 etc. In the virtual console I could type my name at the prompt and press enter, but apart from the cursor moving down a line, nothing would happen. After a while I got 9 or so messages on the console of the form "INFO: task mount.ntfs:2725 blocked for more than 120 seconds", followed by ""echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message". The other messages related to: firefox, Socket Thread, DNS Res~ver (3 times), cron, xfwm4 and getty.

Apart from these messages nothing else changed for the several minutes until I forced a shutdown. I could return the console cursor to the next line, I could switch between terminals and I could move the mouse pointer about, but I couldn't do anything else.

In case it is relevant, the NTFS activity that was fairly heavy, it being a system backup to external disk using Deja Dup.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: linux-image-lowlatency 3.8.0.27.15
ProcVersionSignature: Ubuntu 3.8.0-27.19-lowlatency 3.8.13.4
Uname: Linux 3.8.0-27-lowlatency i686
ApportVersion: 2.9.2-0ubuntu8.3
Architecture: i386
Date: Mon Aug 12 13:25:06 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-08-05 (6 days ago)
InstallationMedia: Ubuntu-Studio 13.04 "Raring Ringtail" - Release i386 (20130424)
MarkForUpload: True
SourcePackage: linux-meta-lowlatency
UpgradeStatus: No upgrade log present (probably fresh install)