Replacing the rear derailleur for the first time
 

Hello.

Last time I was riding (trekking bike), I had a problem with my derailleur. I discovered that the cage was detached from the body (with the attaching screw missing) and was bent in half, hanging on the chain. Now one option is to go to the LBS, but I would prefer to learn how to replace (and choose) the derailleur myself.

Current derailleur: Shimano Alivio, RD-M410
Cassette: Shimano Alivio, CS-HG50-8I (sprockets 11 to 32)

Should I do it myself? What should I look at when I want a more durable RD?

To try youself could be a good lesson and starting point for learning about bike mechanics.

A Deore or Alivio rear mech will probably be a good bet.

Good advice. Once you pick up your rear mech, jump on you tube & punch in " installing / adjusting rear derailleur". THere are some good tutorials on their. Set the high & low, hook up the cable, adj. tension & position and go for a test ride.

It would be wise to figure out why this happened first. If you replace the rear der. and there is an underlying problem, you will be doing it again.

I believe I misaligned the chain when I was replacing the wheel a while ago. The problem hasn't manifested earlier because I rarely shift to the lowest gear.

I'd say go with the same or similar model derailleur so you won't have too many compatibility issues. A used one might be cheap-ish (maybe $20?)
You might also need a chain tool to remove the chain from the cage.
With changing the rear derailleur, you would also have to install the cable into the new derailleur and you would have to adjust it to shift correctly.
I would recommend you take it to a bike shop if you aren't comfortable adjusting the rear derailleur.

Sorry to highjack your thread but i'm in the same situation and I need more info from this point.

I have a busted Shimano acera (RD-M330). How you can make sure you pick the right derailleur to replace the busted one. Can I order a newer model (http://www.chainreactioncycles.com/M...?ModelID=38673) and be sure it will work fine? The older model is long cage also, but smaller pulley like 11 tooth. the freewheel/casette is 8 speed. (11-30)

Pictures? I am having trouble understanding how you could not notice a detached derailleur cage. It would make an awful racket if it managed to not get sucked into your cassette. In fact I can't even imagine how the bicycle could continue to function at all with the derailleur cage separated from the derailleur.