Keeping a backup on the same drive does no harm and can serve a useful purpose (e.g. guarding against accidental deletions of a working copy and restoring a corrupt catalog) except in a case where there is a complete failure of a hard drive. In my experience those are few and far between. But of course they always happen at the worst time. So, I agree with Michel and TSN that it would be best to have a second (and third) hard drive used solely for backups.