how to boot or access problem file system

Michael Powell nightrecon at
Thu Jul 30 19:24:18 UTC 2009

PJ wrote:

> What can be done to access a file system that seems to have the boot
> sector screwed up?

Usually there are more than 1 file system present. The MBR will have no 
bearing on any other than the one you need to boot from, and this is usually 
the "/" - aka "root". Having a screwed up MBR will only prevent a boot and 
generally shouldn't change or cause any corruption to the other file 
systems. Caveat being what occurred that produced the situation in the first 

Look in here for a list of .iso files:

There are the boot-only and a livefs images available. The boot-only would 
be used for a network installation. What you want is the livefs image. 
Download and burn to a CD.

> The /usr files should be ok but how to access?

Boot from the LiveFS CD. There will be a very basic minimum system present 
that contains some tools which may be useful. Once booted you should be able 
to mount the problematic file systems from the hard disk and possibly make 
repairs. It is probably best to utilize the same version as the OS you are 
trying to repair.

> I get errors that the file system is full and I have no idea of how to
> deal with the boot up - the help message is no help!

It may be that you need to locate something you can delete so that the file 
system is now "un-full". 

> Boot says it cannot find a kernel... surely there must be some kind of
> recovery process even if nothing has been backed up. 

There are recovery processes available, but mostly this involves a 
knowledgeable sysadmin and not some magic bullet automated software. This 
skill requires an in-depth understanding of how the OS functions, and this 
can take a while to learn. Along with making some mistakes along the way to 
have something with which to "practice" on.   :-)

> Surely FreeBSD must
> be have something that functions like certain software does on MS ?

Why would FreeBSD be concerned with being like $MS? Going down this path is 
a waste of time. Forget the $MS and learn the FreeBSD. The learning curve is 
initially very steep if all you've ever known is $MS, but if you plug away 
at it you will at some point crest the hill and have a "light bulb goes on" 
moment where all of the sudden a lot of disparate material solidifies into 
something cohesive.  

> I don't have a problem with irrecoverable files, I would just finally
> understand how things work and what can be done on FBSD.
> TIA.
> PJ

Not knowing more details can lead to dangerous advice in this kind of 
situation. It may be something as simple as boot0cfg -B -d 0 is all you 
need. Blindly giving and following such advice without knowing all the 
circumstances may quickly escalate into disaster. An example would be 
something like you are triple booting 3 different OS's with Grub and us not 
knowing that.

You should probably read the man pages for fdisk, disklabel, and boot0cfg 
and see/learn what particular command will extricate you from the situation 
you are presently  more familiar with than us. Get it wrong and it will only 
get worse. But there are at least 3 ways present in those docs alone which 
can be used to write out a new MBR.


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