The problem is that i didn’t mean to write to the hdd, but to a usb stick; i typed the wrong letter out of habit from the old pc.
For that issue, I recommend never using unstable device names and always using /dev/disk/by-id/.
As for the hard drives, I’m already trying to do that, for bigger files i just break them up with split. I’m just waiting until i have enough disks to do that.
I’d highly recommend to start backing up the most important data ASAP rather than waiting to be able to back up all data.