It depends on whether you choose “Managed” or “Reference” style importing into Aperture.

In a “Managed” configuration Aperture copies the original image files into it’s Library folder. After importing is complete you could choose to delete your copies, though I would caution you to hold off on doing so until you have confirmed you really plan to stick with Aperture. You would hate to lose all the work you did in Photo Mechanic if later you change your mind. This configuration makes it easy to copy or move entire Libraries from one location to another because everything is self-contained within the Library including the original image files.

In a “Reference” configuration Aperture leaves the original image files in their current location and only records where they reside. Some prefer to do this as it keeps their Library much smaller. With “Referenced” images, the images can be stored on an external disk that can be backed up separately from where you store the Library. If you move the original image files, you will have to tell Aperture where you have moved them. Aperture does not manage their physical storage location other than recording where it is so it can source the file when necessary.

Regardless of which method you choose, Aperture always stores the image thumbnails, previews, metadata and all of your adjustments in the Library, not in the image files.