I created a persistent USB thumb drive boot with LUKS for drive encryption (250 MB /boot and then 7.5 GB ext3 root). There's no swap partition because this is just to store some private files.

I want to make two or three copies of this thumb drive to store apart in case one gets destroyed/lost.

When I am booted from that thumb drive, can I plug in a same sized drive and just do a

command to make a copy of itself to another thumb drive without booting to another host OS? This way when I make changes to one (such as to update the OS, apps, or data), I could then just copy the entire thumb drive over to the other two while still booted in the original USB.

Or would I effectively have to shut down, boot to my normal host OS (or other), and then copy the thumb drives when not booted into them.

Not a good idea to copy it live. At a minimum when you boot from the new drive it will say that it wasn't shut down cleanly, so will do fsck. Any I/O that occurs during the copy could corrupt the other drive since it may end up partially done. Better is to do a file-level copy using rsync. That also makes updates faster.

Thank you. I went ahead and tried it, and it crashed when booting x-windows.

Really no biggie. But very happy the process works through the host OS. Can save some files offline now and be less worried about corruption/theft.

dd is a good way to make an exact copy but as above, a few gotcha's exist. Never dd off a mounted system drive. dd clones and images may have unique items in grub and other locations that need to be changed. dd is best to exact same media.