Let's look at some of the things in your question:
As soon as I resume the output, the system begins slowing down, and within a minute it is again almost unusable.
This gives a hint that something is backing up over a period of time and eventually can't sustain its throughput. Since you said that this is on a different disk than everything else, it ...
Instead of adding disks on the level of operation system you can do this directly in hadoop. You can add them to the dfs.datanode.data.dir property. The format is
I am not 100% sure hadoop can ...
Slax is a distribution designed primarily for USB-based installations, but it can be copied to the HDD also. If you followed Slax's instructions, there should be a directory named slax at the root directory of your HDD. You should verify that any files there have not been damaged: compare them to the corresponding files on the USB. If you find that a large ...
In Arch's wiki you can find a list of utilities for recovery data. I recommend the use of ext4magic or extundelete, which are optimized for ext.
In turn, there are various tools to automate your backup like tarsnap amd restic.
When you ran dd as the root user you bypassed all the protections that the system gives you.
First, you increased your permissions from a normal user to the root account, which can do anything on the system. Second, you then used the dd utility, which cares nothing about its input or output, to write directly to your external disk.
While I'm sorry to hear ...