Bug 27048

I originally discovered this problem when I upgraded from 4.1.1 to 4.2, and found that I was getting 'hard errors' whenever any process tried to access the hard drive (these occured on boot as well). I then reinstalled 4.1.1, and waited for 4.3 to be released. I just rebuilt the system with the 4.3 code, and found that this same error is occuring. The error is as follows:

The system is still (barely) functional, but I cannot perform any operation which would require a write to the disk (this is not quite true, writes to disk work occasionally). All read operations complete just fine, however.

The following are example of the errors, as found in dmesg:
ad0s1a: hard error writing fsbn 415 of 176-191 (ad0s1 bn 415; cn 0 tn 6 sn 37) status=51 error=04
ad0s1f: hard error writing fsbn 6003135 of 2884288-2884303 (ad0s1 bn 6003135; cn 1488 tn 56 sn 23) status=51 error=04

Fix: 

Perhaps isolate what exactly has changed in the relevant code since 4.1.1, and return it to its original state.
How-To-Repeat: The only way to do this, I imagine, would be to attempt a write to the hard drive on the affected computer. As such, I am willing to give a developer ssh access to the affected computer, and root access if (as I suspect is the case) necessary.