First you need to locate the disks. This can be done by removing the lattice panel on the front of the server. There should be a switch (where the arrow is in the picture) which when pushed, releases the lattice panel.
Once you have done this, you should be able to see all the disks (as seen in the attached picture). The numbering starts from the bottom left, reading left to right and going up. The slots for the vserver 1000 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.