Steam Library-Must be on a filesystem mounted with execute permissions

This Steam error occurs because your drive is mounted by root and you don't have execute permissions on the drive (I think). You can use the disks app to change this.

Search disks in the dash, highlight your drive and under the usage image is a small cog icon, click this and edit mount options, now make your settings look like this

Leave the rest as it is and reboot. Now your drive can be found in /mnt and is auto mounted as your user at boot.

This is how I fix this error in 14.04, NTFS drive.

just got this error despite already having uid and gid set up properly as @delf answer; in my case windows left the "dirty bit", so i had to unmount it, run sudo ntfsfix -d /dev/sdb1 where sdb1 is your disk, and remount it.

For the logs: using ntfs-3g driver, the following minimalistic fstab did the trick for me

/dev/sdaX   /media/target-mountpoint    ntfs-3g defaults,x-gvfs-show    0   0

Hope that helps anyone :) Before i had forced uid and gid (to the user i was actually logged in with) and that didnt worked for some reason (i could create folders and files on the drive from a shell though, not sure what the problem was). So the options that didnt worked:

defaults,permissions,users,uid=1000,gid=33

(Yes, my host user has www-data as primary group)