MyISAMChk deleteing records?

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Hi,
Just wondering if anyone could help me understand what just happened. I backed up a table by inserting its contents to another:

mysql>
mysql> select count(*) from xxxxxx;
+----------+
| count(*) |
+----------+
| 29788 |
+----------+
1 row in set (0.00 sec)

mysql> insert into xxxxx_back1 select * from xxxxx;
Query OK, 29788 rows affected (4 min 59.61 sec)
Records: 29788 Duplicates: 0 Warnings: 0
mysql> \q


THEN, I run myisamchk, and it DELETED THE CONTENTS OF THE TABLE!!!!!!!!!!!

- recovering (with sort) MyISAM-table 'xxxxx/xxx_back1.MYI'
Data records: 29788
- Fixing index 1
- Fixing index 2
- Fixing index 3
- Fixing index 4
Data records: 0




Wow. Data records from 30k -> 0 in 10 seconds.!

!

What is going on!?

jerry