I was importing a slightly larger database into MySQL on a development server running Ubuntu 10.04 and Virtualmin. I kept getting an error 'ERROR 20 (HY000) at line 66: Disk is full writing '.// while importing from command line. Turns out user disk quotas are somehow affecting the ability to complete this! For now I've disabled Disk Quotas in the Webmin System settings. Maybe I'll be able to look into it more in the future.