First you need to locate the disks. This can be done by removing the lattice panel on the front of the server. Note that opening the case by removing the side panel will cause the server to automatically shut down if it hasn't already.
Once you have done this, you should be able to see all the disks (as seen in the picture below). The numbering starts from the bottom left, reading left to right and going up. The slots for the A1000 have been labelled in the attached picture to help explain. If your server is on its side, the numbering makes a bit more sense and starts from the top left corner, working its way down and then along.
To find the disks on a A500, you will first need to turn the server on its side and remove the entire side panel with the air ventilation holes. This can be released by pulling the latch at the back of the server (arrow in picture below).
Once the lid is off, you should be able to see all the disks (pictured below) and their positions within the server. The slots have been annotated in the diagram below. The numbering starts from the bottom right, working its way around the box clockwise.