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I have the problem that my machine turned the root filesystem into readonly mode and remounting it as writeable won't work:

# mount -o remount,rw /dev/sda1 /
mount: block device /dev/sda1 is write-protected, mounting read-only

I want to copy some data away from that system and I attached a USB drive and tried to mount it:

# mount -t ext3 /dev/sdb1 /mnt
mount: /dev/sdb1 already mounted or /mnt busy

/mnt is an empty directory, so mounting there should usually work, but it seems that having this directory on a readonly filesystem doesn't make it possible to mount something there.

I think there is no activity going on for /mnt, since the following command doesn't return any output:

# lsof | grep "/mnt" | grep -v grep

There is no other partition mounted as the readonly root filesystem at the moment. Is it possible to still get the attached disk into the directory hierarchy somehow?

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Is there anything useful in your logs regarding the fact that your root filesystem was remounted read-only? – dawud Oct 10 '13 at 11:16
up vote 5 down vote accepted

The first error message is telling me that you'll need to reboot - there's something wrong with the disk device itself that the kernel can't resolve without a reboot. If you're booted from SAN, look carefully at your boot LUN and make sure there's nothing wonky with it, or the zoning for the SAN, or the export of the LUN.

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Set the mount point to anything that is not on the same partition as "/". For example, often the "/tmp" directory or some other directory is a separate partition. Then, it's as simple as creating a temporary mount point:

mkdir -p /tmp/mnt

and then

# mount -t ext3 /dev/sdb1 /tmp/mnt

AS long as /tmp is on a separate mountable partition, it completely bypasses any issues you're having with the root partition.

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You probably have a stale mtab. Try mounting with the -n option.

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Apart from /dev/sda1, if there is any separate partition available, say /boot, then create a directory under /boot and mount your usb drive there. What I suspect is /mnt is also part of /. If / is ro, then /mnt mount may not work.

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