A trezor can not be infected with virus right? If my personal everyday computer have virus, and I plug in the Trezor, it doesn't infect the Trezor right? But with a external harddrive, it could?
Well, technically a Trezor could be infected like any other computer, it has a CPU [1] so it can be programmed to do different things. BUT(!) its way easier to find a loophole in a complex machine than it is in a slim machine. Its easier to make a mistakes if you write several million lines of code (modern OS) than it is if you write a few tousand lines of code. I am not sure how many lines of code Trezor actually needs to work, but its a different dimension than an operationsystem. The other things is that its more likely for someone to write malware for a broadly used OS than it is for special purpose hardware. If you have to infect a few million machines to find a single trezor, dont bother, see what the few million machines have to offer. Thats why there are so many viruses for Windows and so little for the other systems.
So as everything technical its not 100% safe, but better than the HD. The harddrive can be read as soon as the system its plugged into is under control. Trazor has to be broken seperately. Like a safe behind a metal door vs. a desk behind a metal door (HDD).
I read through the link you attached, but didn't understand the most of the Seeds and Change Addresses part. However, I think I shouldn't even bother to mess with that one, then hopefully, all my coins are safe.
Yep, just keep it in the default setting and you are fine.
And yeah, don't bother with private keys either, if you use a wallet like electrum with a seed. Just keep a copy or two of the seed, and it should be good!
Thats why I like Electrum so much. Armory would require constant new backups. I recently installed mSIGNA its slim but needs a local (or remote) bitcoin core to work with, supports multi sig and all the nice stuff. The userinterface is a bit complex though. *sigh* by the time I found my perfect wallet there will be 10 more to test.
So Tails is a OS. And anything done on Tails, are not stored on my computer. But on Tails, I should only install the electrum wallet, and nothing else, right?
Yes, its usually on a DVD (safer because it cant be modified) or USB (more convienient as it allows to store extra data) and is booted into your memory. Thus it does not access your HD because its not actually installed. As long as you trust the hardware you could use it on any machine, even an infected one. The only problem I currently see is that you have to get electrum once you booted tails, but that might change with the next version [2] of tails.
[1]
http://doc.satoshilabs.com/trezor-faq/overview.html[2]
https://labs.riseup.net/code/issues/6739