I have never seen the insert disk error with TERA. The installer TERA initially shipped with would have been able to throw that error since it was distributed on a set of disks, but that was long before we moved to free-to-play. I've asked our QA team if they have seen that error, but they have not been able to recreate it.

Is is possible you have a mod installed? Have you tried running a error check scan on your hard drive? Are you trying to run TERA from an external drive?