[Klug-general] Solution to media cards mounted read only

[James Morris]

Hi,

I'm posting this in the event that someone finds it useful. It's one
of those problems that I've been working around for ages but finally
had enough of to locate a solution.

I've long thought it was my own fault that SD cards mount read-only,
that some aspect of system configuration was missing.

However, every so often, they do mount with write access but then time
passes and they don't. Again I assumed that system configuration had
changed after updates/distro change etc.

Today I found some claims that read-only mode in Linux is caused by
file system errors on the SD card. Apparently using the card in MS
Windows fixes the problem. But that's too inconvenient and not a
solution I want to use.

The cards I have are not old nor heavily used, so I tried mounting
them in turn and eventually found one which successfully mounted with
read/write mode.

Comparing kernel output via dmesg of the different cards revealed they
had different 'mode sense' values, which seemed to confirm what I read
about flags in the card's file systems which caused Linux to mount
them read only.

This brought me to the search terms 'sdhc "mode sense"' which yields
the solution* which doesn't involve MS Windows.

You need the hdparm utility installed, and also, to know the device
node. Basically you use hdparm to force Linux to re-read the partition
table of the card. For instance, on my system:

hdparm -z /dev/sdd



James

* http://forums.gentoo.org/viewtopic-p-6367579.html?sid=58a500f3e297c47e843871e6c67db21f#6367579

PS Some of these cards are min-sd's inserted into a micro-sd adapter.
The 16gb card had become unmountable in it, the 2gb card was mountable
read-only... I did hdparm -z /dev/sdd on the 2gb, and now have
read-write access on it, but also, on the 16gb too!?