Valve is investigating a particularly nasty bug affecting Steam for Linux that's capable of wiping out both personal PC data and files stored on external storage drives.
As detailed by users on Valve's GitHutb Steam for Linux page, it stems from a line of code in steam.sh, which reads 'rm -rf "$STEAMROOT/". Normally the command is used when you choose to refresh Steam files -- something that can help solve game installation and other problems.
The doomsday event occurs when the code is run after a user has moved the Steam installation directory, which removes the directory part of the code to become 'rm -rf "/'". With no folder specificied, this triggers the deletion of every file on the machine and any connected external drives.
Valve told PC World that the bug had affected a "handful of users", adding that it has been unable to replicate the problem itself and is "adding some additional checks to ensure this is not possible while we continue to investigate".
One of the users on Valve's GitHub for Linux page reported losing all system data, in addition to files stored on a 3TB external drive -- many of which was, luckily for them, backed up to the cloud.
The message is pretty clear: unless you want to risk losing your personal files in a Steam-shaped black hole, it's best to leave the Steam installation directory where it is until Valve has done a bit more investigating.