scheduled autobackup to USB hard drive for windows

Below batch file is an example of taking backup to .bkf file on a USB backup drive.
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> ntbackup backup k: p: /L:s /F "f:\serverback.bkf"
> f:
> ren serverback.bkfserverback-%date:~4,2%-%date:~7,2%-%date:~10,4%_@_%time:~0,2%h%time:~3,2%m.bkf
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Above batch file will rename serverback.bkf file with current date and time.bkf file on f: drive.
In the practical scenario lets say we have 10 GB data to be backed up on daily basis. We can use 250 GB USB HDD (2 nos.) as backup drive. Each can be used every alternate day. This will allow us to retain backup upto 25 working days.