[myTouch 3G, Magic General] mumilover:
Ahh... good question :0)

You need to know a little about cryptography.

You sign things using cryptography. You can also encrypt things using cryptography. When encrypting, the content get changed to something only the intended receiver can read. When you sign things, the content can be read by everybody. You use the signing to verify that what you read has not been tampered with and actually is from you. So if someone "changes" anything in a signed file, cryptography will guaranty that you will discover that. If anybody else is imposing you, cryptography will show that he/she is fake. How does cryptography work then? This can be done using many methods, but here is what android uses (or a simplified version).

Cryptography is done using 2 distinct keys, public and private keys. Public key is what you "give" to everybody you want to communicate with. Private key is yours, and ONLY yours. If i sign an object with my private keys, then all can verify that this is object is from me by using my public key to check the signature. If you sign an object with the public key, then i can check using my private key, that the object is to me and only me.

So when HTC or who ever makes an update, they sign their software/updates with the private key. The device, here a magic, has the public key and can check the signature and verify that the update is from HTC and nobody else. It can also check if you have changed anything at all. If the update has been tampered with or is not from HTC, then the device will reject the software/update. Simple but effective.

Recovery:

The stock recovery will only accept signed updates by HTC. The custom recovery will accept signed updates, which are signed using google test key, which is freely available. (i'm not sure if it's google or somebody else, but you get the point). Clockwork recovery has an option to disable signature check to let you load any zip-file/update you want. The idea of signing the update.zip/rom.zip is to make sure that who ever made it, knows what he/she is doing. If you e.g. load a rom.zip which fails the signature check, then you know 1 of 2 things:

1) the person did not signed it

or

2) the file has been tampered with since it last was signed.

So if you want to load a file which fails the signature check, then you must re-sign it or disable signature check. But i will investigate WHY is the file failing before i re-sign it and load it.

The goldcard will not help here, since the only purpose it has is to allow you to flash an sapphire.zip which is NOT meant for your device id. It will not magically allow you to run code which is not signed.

Even in fastboot or ruu mode. The sapphire.zip is been checked for signature to make sure it's from HTC and has not been tampered with.

So, NO you cannot take FroyoRider_FeeyoCM61_6.35_34_v2.1.zip and rename it update.zip and try to load it using stock recovery. Unless you sign the files with HTC private keys, which make you a very powerful man and i would like you to contact me :0)

I hope i have shaded some light to your question!

ps: Some of the OLD guides will have you to rename the files like the "FroyoRider_FeeyoCM61_6.35_34_v2.1.zip" to "update.zip" and load it from custom recovery. That's because in the past custom recoveries did not have a "flash zip file" option which will let you choose any file ending with .zip!

That's is what DonJuan692006 is referring to.