A few days ago I started noticing this error in the backup logs for MySQL:

mysqldump: Couldn't execute 'SHOW TRIGGERS LIKE 'smf\_log\_actions'':
Can't create/write to file 'C:\Program Files\MySQL\MySQL Server 5.0\
Data\#sql_654_0.MYI' (Errcode: 17) (1)

mysqldump: Couldn't execute 'show fields from `mos_banner`': Can't
create/write to file 'C:\Program Files\MySQL\MySQL Server 5.0\Data\
#sql_654_0.MYI' (Errcode: 17) (1)

Turns out it started directly after installing McAfee Enterprise 8.5. I found this bug report on MySQL's website: http://bugs.mysql.com/bug.php?id=25872. The bug has been closed because it has been determined to be a problem with McAfee and not MySQL.

The quick-fix is to move the MySQL temp directory away from the Windows temp directory and exclude the new MySQL temp directory from McAfee on access scans. I haven't found a permanent solution yet but I will post it when I do.