How do I copy files from various locations to a USB flash drive in Single User Mode?

I'm unable to boot normally or in safe mode. When I try either method, the progress bar and pinwheel appear for a while, but once the progress bar reaches about halfway, the system just shuts down. I've also tried repairing the disk in Recovery Mode, but I get a message telling me it's not possible. Verbose mode comes up with a couple of pages of missing threads, which is probably the reason why. The only option I see left open to me is to try and copy the most important files onto a flash drive if I can - hence Single User Mode being my last resort. The friend I'm trying to help (no, it's not me 🙂) didn't back up their files.

Ok, I'll have a look through the Terminal commands regarding mount and mkdir. Where or how would I find out the name of the /dev/disk? Would it be the long identifier I mentioned earlier which appears when I plug in the flash drive - i.e. USBMSC Indentifier (non-unique): 070B54A355.... or should I try using the text name that appears when I plug it into my own computer?

Many thanks. I'll give that a go and see how I get on.

Hmm, I tried using /dev/diskmsn on all the disks listed by /bin/ls /dev/, but I keep getting a message saying "permission denied". The list is quite long, but the rest all have names such as ttyve rather than diskmsn. I also tried on the listed rdisks, though without success. The flash drive isn't locked, so if it's among those listed, I can't see why permission would be denied. I'm pretty stuck.

As far as I'm aware, I'm in Single User Mode: CMD + S at boot. Anyway, I managed to get a bit further. I verified that the flash drive is called disk1s1 by using ls -l /dev/disk* to give me a list of available disks and fstyp /dev/disk1s1, which confirmed it was MSDOS. I tried repairing the disk several times using /sbin/fsck -fy without success. At this point, realising I still had no write persmissions and with disk still not verified as OK, I tried to mount anyway (probably not a good idea in hindsight) using /sbin/mount -uw / which deleted some orphaned files and folders. After exiting, I found I was able to boot to the login page. Now I only need the password from the familiy member of the friend I'm helping and I'll be able to find and transfer his files and set about either repairing the disk or doing a reinstall. Thanks for all your help. I'll post again, should I run into any further trouble.