Export Aperture photos to external drive on old Mac

Assuming you have a version of Aperture that still runs:

Here's a copy of the Aperture manual, at the Exporting page

https://apple.manymanuals.com/software/aperture/user-manual-43043/291

I link this because while the basic commands are simple File -> Export, there are many options depending on what you want to export (Originals or Edited versions) and at what settings (File type, size, quality etc etc)0

Weich version of Aperture have you been using on the old Mac? Aperture 2 or Aperture 3? Aperture 2 came out in 2008, Aperture 3 in 2010.

How to export from Aperture is very similar in both versions, but the structure of the library is different. Anyway, you may want to save a backup copy of the Aperture Library. You can open an Aperture 3 Library in the Finder - just ctrl click it in the Finder, then use the command "Show package content" and find the original images in the subfolder "Masters" arranged by the year, month, day import and the previews of the edited edited versions in the subfolder "Previews". Just be careful not to modify the library when you browse it in the Finder. I don't remember the structure of the Aperture 2 library, however.