You don't need to cd to a directory -- any file-oriented command will accept a full path name just as well as a plain file name.

Code:

mkdir -p /root/.ssh  # -p makes it not do anything if the directory exists already
cp /mnt/BootDisk/authorized_keys2 /root/.ssh/
chmod 600 /root/.ssh/authorized_keys2

If you want to avoid repeating the path name, use a variable to abbreviate it.

Code:

p=/root/.ssh
a=authorized_keys2
mkdir -p "$p"
cp /mnt/BootDisk/"$a" "$p"/
chmod 600 "$p"/"$a"

This way, it's also easier to globally replace the path name if you ever want to do that -- only one place to change.

PS. Since this question is by and large unrelated to your original question,. perhaps you should have started a new thread.