It would be more understandable if you put each command on a separate line. The first time I read it, I thought your ran "mkdir /mnt/usb 2".
Repeating the process and cutting and pasting the commands would provide us with the exact commands & error messages.
You should indicate what filesystem is on the usb stick. If it is fat32, which is the most common, use the filesystem vfat when mounting. You need to add options in the mount commands to change the ownership & permissions during the mount. E.G. If your username is "rkarmakar", you might use "sudo mount -t /dev/sda1 /mnt/usb -o rw,default,uid=rkarmakar,fmask=0110,dmask=0001".
I can't imagine that a usb stick would have the device node sda1, unless you are running an older kernel that uses the device node /dev/hda for the first pata disk.
Also remember that /root is for the root user. You can't copy files there as a normal user, and shouldn't be messing around with system directories without good reason.