A blown carriage belt can ruin your day and turn your plotter into a giant boat anchor! I explain this to many callers every day...with the 400 and 700 series Designjet the original carriage belts are just at the age where the rubber is sure to break down in the near future. Just like hoses and belts on classic cars, they must be replaced. There is one sure way to know if you are looking at buying an older HP Designjet: Look for little black fragments on the silver floor under the belt. It may look like a mouse has been in there...or it may just have a fine black dust. If you have black dust, you probably still have a number of plots you can run...if you have black chunks, don't expect to finish a plot without having your carriage belt blow up, ok?
If the floor under the belt looks like this you'll be needing a new belt very soon!