You can make a copy of the MyISAM table files *with running mysqld*
a) the table has been flushed to disk and
b) the table is not written to while you copy files
both can be accomplished by FLUSH TABLES WITH READ LOCK. Another way
would be LOCK TABLES ... READ followed by FLUSH TABLE ...
After you finished copying, just UNLOCK TABLES.
During copying, either all tables or at least the table in question
would be locked, allowing read only operation - all writes would have
to wait until you unlock the table(s). This is not perfect, but
probably much better than mysqld being completely offline.
This could be done with running mysqld as well
1. create empty copy of your table (CREATE TABLE ... LIKE ...)
2. rename tables, effectively swapping old and new table
(this is an atomic operation, even for multiple renames)
3. flush the old table
4. copy the files
5. drop the old table
Axel Schwenke, Support Engineer, MySQL AB
Online User Manual: http://dev.mysql.com/doc/refman/5.0/en/
MySQL User Forums: http://forums.mysql.com/