The /etc directory is normally always there. No need to mount it.
Check to see if the files you want are already there first:

ls -l /etc/rootcert/*.der

"ls" is the "list files" command, like "dir" on MS-DOS. The "-l" asks it to produce "long" (detailed) output.

If you see a bunch of .der files there, then copy them to the USB-stick similar to before, just omitting the "mount" and "umount" for /etc/..