How does one erase a FreeBSD disk (slice) label?

Dan Strick strick at
Tue Aug 19 01:41:54 PDT 2003

How does one erase a FreeBSD disk (slice) label?

Suppose I want to reuse a disk slice that currently contains a FreeBSD
file system for some other purpose that would not involve a FreeBSD
disk label and partition table.  The kernel won't let me overwrite
the label block.  Even if I write enable the label block with the
DIOCWLABEL ioctl or the "disklabel -W" command (which is not as simple
as you might think*), the kernel will insist that whatever I write into
a label block be a valid label.  For example, it will reject a zero block
because it doesn't have the correct magic number for a disk label.
It seems that once you create a disk label, you can't get rid of it.

I tried changing the slice starting offset with fdisk so that the
label block was someplace else and then zeroing the disk sector,
but that didn't work.  (I don't know why.)  When I restored the
slice the label somehow reappeared.  What do I have to do to get
rid of the blasted thing?  Boot MS Windows and FORMAT the slice?
Disable the code in /sys/kern/subr_diskslice.c that protects
disk labels and build a new kernel?  Go back in time and kill
the person that wrote this code before he wrote it?
Learn to love penguins?

Please help ... I am slowly going crazy...

Dan Strick
strick at

*P.S. An example of the extent of the insanity: the "disklabel -W"
command does a DIOCWLABEL ioctl to write enable the slice label.
It used to be the case that the label write enable flag was cleared
on the final close of the special file.  (Perhaps it still is.)
That meant that the effect of the "disklabel -W" command lasted only
until the disklabel program exited.
The workaround was to do something like "od /dev/da1s2 | more"
before "disklabel -W da1s2" in order to delay the final close.
Of course this was documented nowhere that I could find.
Alas, that hack is no longer sufficient.

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