According to Intuit, Error Code -6000, -304 can occur when a user is attempting open a QuickBooks company file (.qbw) and either there’s not enough disk space to do so or disk quotas are turned on and the user account running QuickBooks has reached its quota.
To resolve this error, relocate the company file (.qbw) to a larger hard disk, turn off disk quotas, or increase the disk quota of the user account running QuickBooks.
For more information on resolving Error Code -6000, -304, consult this Intuit knowledge base article.