Update: I managed to install a new firmware version as described in https://bugs.launchpad.net/linux/+bug/75295/comments/97 . Since my last reboot hard disk accesses did not freeze any more. So this might have been sufficient.
Update: I managed to install a new firmware version as described in https:/ /bugs.launchpad .net/linux/ +bug/75295/ comments/ 97 . Since my last reboot hard disk accesses did not freeze any more. So this might have been sufficient.