How to mount a hard disk as read-only from the terminal

You do not mount /dev/sda, that refers to the entire disk. You mount /dev/sda1 or whatever partition you want.

Make a mount point, call it anything you like.

sudo mkdir /media/2tb

Mount

sudo mount -o ro /dev/sda1 /media/2tb

When your done, you should unmount the disk

sudo umount /media/2tb

See man mount or https://help.ubuntu.com/community/Fstab

When mounting the filesystem read-only, some trouble may happen. The system may try to write into the device anyway and fail.

For that reason the noload flag may be used, to notify to the system that the disk is blocked.

The best solution I found was:

sudo mount -o ro,noload /dev/sda1 /media/2tb

The manual of mount(8) explains this options as follows:

-r, --read-only

Mount the filesystem read-only. A synonym is -o ro.

Note that, depending on the filesystem type, state and kernel behavior, the system may still write to the device. For example, Ext3 or ext4 will replay its journal if the filesystem is dirty. To prevent this kind of write access, you may want to mount ext3 or ext4 filesystem with ro,noload mount options or set the block device to read-only mode, see command blockdev(8).

[…]

norecovery/noload

Don't load the journal on mounting. Note that if the filesystem was not unmounted cleanly, skipping the journal replay will lead to the filesystem containing inconsistencies that can lead to any number of problems.

For more info see the great explanation in “How to Mount Dirty EXT4 File Systems” on the SANS Digital Forensics and Incident Response Blog.

I am plugging a USB connected drive into Ubuntu 12.04 and the system is mounting it automatically. In Terminal, if I just say mount it shows me the current info. I want to remount it read-only.

Extrapolated from man mount(8):

sudo mount -o remount,ro /dev/sdb4 /media/HP_TOOLS

Seemed to work nicely. Had to do it for each automounted partition.