Congratulations on the backup - not many have such before they come asking for help.
Absolutely essential when screwing around with the system. Looks like you'll get plenty of pertinent practice at least ....
You had the correct syntax
- you just picked the incorrect target (drive).
You may not be asking what you think you are asking, and even if you are you may not be asking the correct question(s). I'll try to be as flexible as possible with the answers.
1). This is not trivial from CLI. Operating systems present (and mount) partitions not devices (generally) - this applies to both Windoze and Linux. When you plug in the drive you should get notified, if not try "dmesg | tail -n 20" (no quotes - on any of my example commands). That should allow you to guess the drive - these days it'll be something like /dev/sd? (one character a/b/c ... ignore digits at end). "ls /dev/disk/" will tell you your drives, but will give no indication of whether they are in use.
2). The command was basically correct - as you indicated you hit the wrong device. The device may be named differently in different distros (Fedora may call it /dev/sda for example). So it's tough to give absolute answers. "dd" is both powerful and stupid - it'll do as you tell it to. No questions asked - online or offline, makes no difference.
3). Ubuntu mounts partitions
. Use "df -hT" to see mounted partitions (correlate with the dmesg output), then use "umount /media/<whatever>" on all the entries that correspond to the device of interest. This time there will be a character (as above) plus a number for the partition(s) - I'm guessing on /media/... as I don't have a Ubuntu handy.
Given what you are doing, it is possible there won't be a partition on the USB at all, but that's a whole different can of worms.
From a GUI you should be able to right-click each partition to unmount it. I rarely use one for such.