I think I have the solution despite the fact that the 3 drawings on the warranty paperwork have no explanation of the drawings or what they are for.
I bought the 1 Tb MyPassport for Mac and believe it is about 1-2 mm thicker than the 500 Mb version. If you put the thicker, 1 Tb, drive in the case it is very hard to try and close the case with the rubber bands that are fitted to the inside lid of the case when purchased.
In other words, I think the case, as supplied when purchased, is fitted for a thinner 500 Mb drive.
The existing rubber bands are glued to two hard plastic strips that are clipped into place in the lid. Drawing 1 on the warranty sheet shows a screw driver being used to prise off these two hard plastic strips complete with the rubber bands glued to them. Drawing 2 shows the two hard plastic strips complete with the rubber bands glued to them being removed from the case. Drawing 3 shows the second pair of rubber bands supplied in the plastic bag having their adhesive cover being peeled off the back of the rubber band and the band then being placed in the same location inside the lid where the original hard plastic/rubber bands were located.
By following these steps you have the case ready for use with a 1 Tb drive. What you have done is removed the the hard plastic pieces which are the thickness equal to the difference between in thickness of the 500 Gb and 1 Tb drives and still have two rubber bands in place to support the 1 Tb.
A long explanation but hopefully everyone can understand. Pity WD hadn't given at least a statement of the prupose of the 3 drawings.
I would appreciate if someone could confirm I am right.