MYSQL+MyISAM+Need information regarding check table statement
Big table?  It should be read-only.  If it finds something, CHECK TABLE suggests that you run REPAIR TABLE -- which will modify the table.
Whenever the server crashes (power failure or whatever), CHECK TABLE is needed for MyISAM tables, but not for InnoDB tables.  I recommend you change Engines.
Since almost every action locks the whole table in MyISAM, I would guess that CHECK does, too.