trouble backing up large mysql database

I have a wordpress MU database with something like 10,000+ tables for various user's blogs. I need to upgrade wordpress MU to newest version, but want to backup the DB before hand.

PHPMyAdmin fails to even load the page when i click export. Ive tried going into the server (windows) and using dos command line:

mysqldump -u USERNAME -p PASSWORD> BACKUP.sql

but it hangs for a minute and gives me the error:

error 23: out of resources when opinging file '.\USERNAME\wp_1037_links.MYD' (Errorcode: 24) when using LOCK Tables

What am i doing wrong, or should i be doing?

**Note that this is not my site, so any suggestions as to the setup of the DB ill have to run by the owner. Im just here for WP related crap, this is kind of out of scope for what i was brought on to do.

From serverfault Patrick

• Try the following: mysqldump --lock-all-table -u USERNAME -p PASSWORD > BACKUP.sql

  Be aware that if this is live, it may stop the sites functioning.

  Patrick : Yeah it is live, any way to do this without disrupting the site?

  AliGibbs : Unsure what version of mysql you are using, but take a look at mysqlhotcopy, http://dev.mysql.com/doc/refman/5.0/en/mysqlhotcopy.html
  From AliGibbs