When you start it up it'll give you some options. Just hit enter, then enter again when it asks for your keyboard settings. After a couple minutes you'll get a command prompt.
First plug in your external storage drive. Wait a few seconds, then type this:
That will list all your hard drives. You need to identify your C: drive (probably /dev/sda) and your external drive (probably /dev/sdc) - it should be obvious from the sizes but if your external drive is the same size as you D: drive you can verify it by unplugging it, hitting (up)(enter) to run the command again, and see which one appeared or disappeared.
For this example we'll say your external drive is /dev/sdc.
Next you'll need to find the partition number. Do this:
It will give you a table listing /dev/sdc1 and maybe some more partition numbers - it's probably just /dev/sdc1 unless you created a bunch of partitions yourself, so I'll use /dev/sdc1 for this example.
Next you mount it (it's like giving it a drive letter):
mkdir /external
mount /dev/sdc1 /external
Finally you dump the C: drive:
pv /dev/sda > /external/dump
If that gives you an error after the first few GB are copied then do this instead:
pv /dev/sda | split -a 5 -b 1G -d /external/dump
Make sure it completes with no errors, then:
And you're done.
That dump will be a complete raw copy of your hard drive. Once we have it we can start looking for your wallet without worrying about losing anything.