These instructions cover configuring a NTFS disk containing Steam games, that was previously used in a Windows environment, to work with Proton on Linux. This allows a user to use the same files to play games on both Windows and Linux without needing to reinstall games for each operating system.

Create a mount point for the NTFS game disk:

$ sudo mkdir /media/gamedisk

Find the User ID, Group ID, attached disk partition, and the UUID using the following commands:

User ID

$ id -u

Group ID

$ id -g

By default, both should be 1000

Attached Disk Partition

$ sudo fdisk -l

It should be labeled similar to /dev/sda2

The trailing letter and number (a2) will depend on how many disks are attached.

UUID

$ sudo blkid

Find the line where the first column matches the label of the fdisk command.

For example, /dev/sda2 would match this line:

...
/dev/sda2: UUID="38CE9483CE943AD8" TYPE="ntfs" 
...

Copy the UUID.

Edit the fstab file to mount the partition:

$ sudo nano /etc/fstab

At the bottom of the file, add the following line (changing UUID, uid, and gid where needed):

UUID=38CE9483CE943AD8 /media/gamedisk ntfs uid=1000,gid=1000,rw,user,exec,umask=000 0 0

On Ubuntu, as long as ntfs-3g is installed using ntfs as the filesystem type will work

Reboot the computer for the changes to take effect:

$ sudo reboot

If the partition is mounted as read-only after reboot

If Windows is installed on the NTFS partition, the Windows Fast Startup feature can cause the mount command to fail. To prevent that, consider to disable it. Example tutorial: PassFab: Disable Fast Startup

THERE HAS BEEN A REPORT THAT THIS MAY CAUSE DATA LOSS

Due to the nature of NTFS, creating files/folders with names that are invalid on Windows will cause disk errors (leading to games that don't launch). The most common issue is a : (colon) character in filenames that Proton creates on the NTFS disk.

Fixing this is pretty simple: create the /compatdata folder on the mounted NTFS disk as a symlink that points to a folder on a Linux partition.

Creating the symlink:

$ mkdir -p ~/.steam/steam/steamapps/compatdata
$ ln -s ~/.steam/steam/steamapps/compatdata /media/gamedisk/Steam/steamapps/

If the /compatdata folder already exists on the mounted disk BEFORE the symlink, DELETE IT!

Sometimes case-sensitivity can be an issue and lead to files not being found, because their names differ in case from what is expected. In such cases, mounting the NTFS partition with lowntfs-3g may solve the problem (source: https://serverfault.com/questions/901855/ntfs-3g-ignore-case). To do that, edit the entry in /etc/fstab as follows:

UUID=38CE9483CE943AD8 /media/gamedisk lowntfs-3g uid=1000,gid=1000,rw,user,exec,umask=000 0 0

And then reboot.