You cannot boot from an external usb hard drive and you don't need to boot from it to use it as a backup drive. What computer model and what operating system? Always mention those in your posts.

Just plug in and windows will recognize it. You can and should assign a recognizable letter designation using a later letter in the alphabet such as Drive R or S or T etc so it doesn't conflict with the operating drives. Use Disk Management to assign letter.

Thanks Mary.  Computer is a Dell laptop, Inspiron 15 , model 3542

I have since found out that Windows can't be booted from a USB drive. So, I will use an external drive for backup purposes and assign the drive letter as you have suggested.