There may be any one of several different reasons why your mp3 player does not allow you to add more music. Don't always assume it is because of write-protection, only if your computer tells you that it is.
1. Write Protection.Your mp3 player's hold/lock/padlock switch may be on. Even if you think it is off, try again with the switch flicked the other way.You may find that puting your mp3 player into a USB port on the back of your computer prevents you from editing it. Try a few different ports.You may have parental controls on your computer. In theory, your parents may have put them on to prevent you from selling illegal burned CDs, but most of you wouldn't so get the controls off.
2. No More Storage.You may just have too many tracks crammed into too small a space. Take the ones you are fd up with off. Or, if your mp3 player is 2GB or less, (ie: 2GB, 1GB, 256MB or, hell forbid, 128MB) get one with four or eight gigabytes of storage space.If you have been using it to store school or work files, like publisher documents, or large digital photos, then You should take them off. Mp3 players don't play pictures or documents, so get them where they need to be going, then take them off. It may be agood idea to buy yourself a Memory Stick with just one or two gigabytes on for files.Some CDs come with silent, short tracks on. Don't even rip these to your PC or laptop.
3. NIP - Not In ProperlyYou haven't pushed it in far enough. Don't push it so hard it feels like it will break, if you have to do that, try another port.You have a piece of foil/playdough/rubbish stuck in your mp3 player's USB part. Dig it out with a Kirby grip, not a paperclip. Paperclips might scratch the inside, but as long as the Kirby grip still has the customary paint blob on the end, it can't do too much damage.Your USB port is faulty. Try another. If it won't fit it into any of them, make sure that you are trying USB ports, get a second opinion, then call an expert if one is needed.
That should cover every reason why your mp3 player isn't responding properly. I'm going to protect the whole answer, so if none of this solves your problem, or you want to add something, post it on the discussion page and I'll put it in as soon as I see it.