write-protected usb flash drive

[Polytropon]

On Sat, 24 May 2014 16:22:59 +0100, Arthur Chance wrote:
> OK, thought I'd better try my own advice rather than just handing it 
> out. I put a microSD card out of an old phone into a SanDisk mSD -> SD 
> adapter and plugged that into my SanDisk SD -> USB adapter, mounted it 
> (FAT32 file system already on it) and wrote a file to it. Worked as 
> you'd expect. I then unmounted and unplugged it, flipped the write 
> protect switch and tried to remount. Result was
> 
> mount_msdosfs: /dev/da5s1: Input/output error
> 
> Mounting it read-only was fine. So, the write protect is honoured by at 
> least some SD -> USB adapters.

This is already on file system level. It _should_ work the same
at upper layers, for example when using dd to write NULs to the
device with the write protection on - an error should (correctly)
occur in that case.

When a r/o mount is forced, the routines accessing that file
system cannot avoid the write protection. Still writes are
possible _aside of_ the file system which should be prevented
by the switch as well. It's probably a good idea to check that
too, e. g. put in the card with write protection on and then
try dd or newfs on it.



-- 
Polytropon
Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...