Q:

How can I automatically set write permissions on mounting a usb drive in linux?

When I mount an external usb drive on linux (CentOs4), the permissions are by default set to read-only. Since there are multiple users on the computer who need to use the external drive, I want everybody to have rw permission for the entire drive. I also want them to be able to mount the drive if the computer has accidentially been shut down. They can use sudo mount to mount the drive, but this will only give them read permission, and I obviously don't want to allow sudo chmod.
Is there a default setting that I can change so that every new external usb disk automatically gets rw permissions?

A:

To enable everyone rw access, the key is umask=0 option to mount command.
sudo mount -o umask=0,uid=nobody,gid=nobody /dev/something /mnt/somewhere

umask=0 is enough, uid and gid just for sake of clarity, so you don't see more 'root' owners than necessarily.

@Tom's answer (writing /etc/fstab entry) will allow you to skip sudo and if you write umask=0 as additional option there, you'll get best of both worlds:
Having this in /etc/fstab:
/dev/something /mnt/somewhere auto users,noatime,umask=0 0 0

allows you to just run
mount /dev/something

and everyone has access to all files.

Here's technical note, if you wish to know details:
As man mount says, 'umask=0' will ensure that no additional rules apply to files access mode. For FAT filesystems (which are most widely used on USB disks), there's no access mode stored. But your current process has some umask value set, you can see it if you run just umask in terminal. mount uses that as default and removes access mode of your umask value from all files on mounted disk. Most widely used umask values are (octal) 022 - no group and other write, and 027 - no group write, no any other access.

