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I am happy to help if I can.

Do you have any idea how the user got your password?
Did you enter it somewhere? It could have been a long time ago, some people collect it first and then use it all at once. However, it seems that this is not the case here.

I think the hacker got access in my computer, while I was performing a backup on my external drive, and for the time the (possible) rootkit gave him access to my computer (in real time), he made a copy of a lot of information that were accessible from my external drive - one of them being a text document with all my keys that usually stays on that external drive. With that in their hands, the rest is quite easy to figure out.
But how and where I got a malware in my computer, this I'm truely unsure.

Sorry for your lost. and the one think i found strange is that lot of inactive accounts are being drained to sepa. So, if the user is inactive here on Hive, must be a very large scale scam.

This is one of my concerns, because even though I left it on an external hard drive, at some point we will need to connect it to the PC, and if it is compromised... the end

Something I usually do when I update these backups is to disconnect the PC from Wifi, trying to minimize any spying at that moment, but I also don't know if this would be effective depending on the spying tools used.

The safest remains paper.