Even when running from the live CD / Pendrive, you must prefix your
chmod command with
sudo. So your steps will be like the following:
- boot from a live CD / Pendrive
- check whether your disk was already automounted (and where to). If not, mount it (see below)
sudo chmod 0755 <path> to adjust the permissions
How to figure out where your disk is mounted: from a terminal window, run
mount (without arguments). This will list up all mounted devices. Check for the
type listed -- you can skip everything not using a "real file system" (your disk probably uses either ext3 or ext4 -- you can for sure skip things like proc, sysfs and the like). If something sounds promising (looking like
/dev/sda1 on /media/sda1 type ext3), check its contents using
ls /media/sda1 to see if it's that.
If it is not mounted, you can check with the
/dev entries where the disk could be (using
ls /dev/ |grep '/dev/sd to check for available devices; your disk should look like
/dev/sdbX or the like -- with X being a number). Compare this with the list of mounted devices. If it's not there, try to mount it and check its contents (as shown above). To mount it, first create a mountpoint, e.g.
sudo mkdir /mnt/mydisk, then try to mount the device using
mount /dev/sda1 /mnt/mydisk and check its contents using
Once you get the right disk there, you can go to change the permissions back on your usr dir:
sudo chmod 0755 /mnt/mydisk/usr.
Now you still might be in trouble if you originally ran the
chmod command recursively, using the
-R parameter. In that case you can either try to fix each entry manually -- or you can go straight for a fresh install...