I guess that if you cannot see what you are doing, then you will not be able to figure out how to erase the HD.
Bear in mind that you cannot erase the HD as long as you are booted from it. The system will not allow it.
Unless you have something illegal on the drive, that is immediately apparent, I would not be too concerned that someone is going to see them. If they were that secret, one would assume that you would have them locked down in the File Vault. The Apple tech who repairs your machine has far more important things to be doing than rummaging around your HD looking for "secret" stuff.
In some states, a computer tech is legally bound to report certain things to the local law. Pirated copies of software and downloaded MP3's do not fall into that category.
P
My craptacular ibook is now on it's second logic board asking for it's third. I've already scheduled a repair with Apple, however, I am reluctant to send in my notebook. I have files that I don't want anyone (a sneaky Apple technician for example) to see and possibly copy or steal. Call me paranoid if you must but,
Is there a way to wipe the drive or delete the files when I cannot get a screen image?

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