How do I access files, copy them on an external hard drive, and see them on it?

I'm trying to recover the password of an old iMac, couldn’t boot to Recovery though. So I went into single user mode to access the files, which worked. I have an external hard drive that is formatted as Mac OS Extended (Journaled) to put the files I'm looking for on the iMac into the drive. I copied them and I went to my computer to access it. But now my drive isn't mounting! When I did the copy I ran the command cp /dev/disk1s2. When I try to mount it on the other computer to see the files, I tried mounting it through the command line with diskutil mountDisk /dev/disk2 and diskutil mount /dev/disk2s2. Both commands gave me an error saying that the disk failed to mount. I also tried mounting it through Disk Utility but nothing happens. I need those files because it contains the password of the computer. How am I going to see the files?