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I need to restore a directory on a Linux (CentOS) file server from an external USB drive. It's the only copy, and I don't want to risk any of the data being altered on it by some mistake with rsync or whatever.

How can I mount an external USB drive on Linux in read-only, just for the temporary purposes of copying the files off of it?

This is how it looks normally in /etc/fstab:

/dev/usblaciexl1        /backup     ext3    noauto,defaults 0 0
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up vote 1 down vote accepted
mount /dev/usblaciexl1 /backup -o ro

should be sufficient.

You can also do (do not confuse if and of, read man dd carefuly)

dd if=/dev/usblaciexl1 of=/home/user/backup
mount /home/user/backup /backup
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Just add ro to the mount options. Something like:

/dev/usblaciexl1        /backup     ext3    defaults,noauto,ro 0 0

See the mount man page for the (huge) list of other mount options.

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