Hello!

Simply question: I want to make a backup on a USB stick in case my hard drive crashes. Do I simply copy the default_wallet file (I have linux) to the USB stick?

Cheers and thanks for your replies.

[Mining]

Help me educate myself so I can educate others  

I watched this video http://www.youtube.com/watch?v=Lx9zgZCMqXE which I think is excellent.

I have some specific questions remaining:

Mining

I guess a miner is not always a node (e.g. when he joins a pool) but a node is always a miner? My understaning is that you are a node as soon as you support the network with the blockchain database running the bitcoind client and update that database.

I also heard the term "full node". Do half nodes exist?

Do miners run specific mining software? Can mining be enhanced by specific mining software? Or are you just as likely to find a block with the bitcoind client? (I am not talking of mitigating the risk by joining a pool)

Transaction validation

The following questions are based on my understanding that the miner that finds the next block defines which transactions are included in this block (if he is a honest miner he takes those transactions into the block that have the higherst tx fees). See 13:00 Correct me if this assumption is wrong.

So the question is then how does the network actually control that the transactions chosen by the miner that found the block are actually valid (enough input, no double spend, --> do other reasons exist why a tx would be invalid?)?
- Is it that the client/network doesnt let me broadcast transactions that I am not entitled to based on my inputs in he first place? If so how does the client / network  know this?
- Or does the puzzle and its solution depend on the tx messages? That would make sense to me

My understaning of the term "transaction message" = all the transaction specific details: Input and output address, btc amount, what else?

How many transactions does a miner have to take into a block?

How is it possible to send the same bitcoin twice at the same time when trying to do a double spend attack? See 10:03

What's the deal with getting the hash below a certain number (see 15:00)? My of the "puzzle" to be solved was something like: All the letter of a sentence that makes sense are mixed randomly. No a computer tries all different possibilities so that the sentence makes sense again. This wouldnt work of with sentences because whether it makes sense if up for interpretation...