Dien Duong wants to back up to two separate external hard drives. But Windows' built-in backup program won't let him do that.
Backing up is all about redundancy. And the more redundant, the better. If some ransomware steals your important files, the last thing you want to discover is that your backup drive is dead. Or that a bug in your backup software won't let you restore anything (that actually happened to me once, a long time ago; and no, I don't want to talk about it).
So Dien has the right idea--you should back up twice. His mistake is trying to do it with Windows' built-in backup tool--called Backup and Restore in Windows 7, and File History in Windows 8.1 and 10.
You could do it, however, with almost any third-party backup program. For instance, with EASEus Todo Backup (either the free or paid version), all you would have to do is create two separate backup tasks. When you want to run one, plug in the appropriate external hard drive and select the desired task.
Windows' own backup tool, on the other hand, can't handle multiple tasks.
While two backups are better than one, two backup programs adds even more protection. You can use Windows' own Backup and a third-party program such as Todo. Or use Todo and another third-party tool, such as AOMEI Backupper. If there's a problem with one of the backup programs--say, it doesn't back up an important folder by default, and you didn't catch the problem--chances are that the files you need will be in the other backup.
Another option would be to do both a local and a cloud backup. You can check out my 3 easy backup steps for the advantages to both types, as well as for general advise on backing up.
One last thing: Some programs now offer both local and cloud backup. For the reasons discussed above, I recommend you use a different program for each type of backup.
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