as long as nothing in there says they actually logged in you are fine.
welcome to the internet
that is why you have linux, so it won't get hacked.
I have blocked a few subnets from china and taiwan on my firewall, since then it has cut the attempts in half. the other thing you can do is change your ssh server from port 22 to another like 220022 or something.
now back to the hard drive, if you plug that in the immediately check the end of the messages file and post what you get related to the drive.