Getting .myi errors?

A .myi database error (for example: "couldn't open thread.myi") usually means a corrupted database. Usually, to solve it, run the following query:
[sql]
REPAIR TABLE tablename;
[/sql]
...where tablename is the name of the .myi file, sans the .myi extension itself. If this fix doesn't work, ask your host.

The Self-Healing Database hack can fix many of these errors as well (although "fix" is relative...it's like taking pain reliever when your arm is broken rather than just healing the broken arm).

Warning! When a table gets corrupted and you run this query, often a row or two will get deleted. Unfortunately, this row can be your user account, a critical thread, etc. Let this have been a lesson to you to always back up at least daily!

[Erwin]

If your forums is large, and you don't backup at least daily if not 12-hourly, then you are asking for some serious trouble in the event of a database crash.