Marios Papadopoulos stores media files on a 1TB external hard drive. He asked about using the same drive for backup.
Technically speaking, there's no reason why you couldn't use the same hard drive for backing up your internal drive and storing overflow data that don't fit on your internal drive. But doing so is a really bad idea.
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First of all, if there are files you're storing exclusively on the external hard drive, those files need to be backed up. Remember the two primary rules of backup:
There's another issue: A backup drive should only be plugged in when you're backing up or restoring from a backup. Why? Because the same fire, flood, burglary or malware that destroys your main, internal storage could also destroy the backup.
Ideally, your internal storage should be large enough for all of your files. If it isn't, consider one of these options:
Install a second internal drive. Once it's installed, copy the overflow files now on the external drive to their new home. Use the external drive exclusively to backup both internal drives.
You'll find this considerably faster and more convenient than storing files externally.
But you'll need a PC with a spare drive bay. That's common in desktops, but rare in laptops.
Replace your internal drive with a bigger one. This is also fast and convenient, and it solves the drive bay problem. But it takes more work to set up. Luckily, I've already provided instructions.
Use two external hard drives. Use one external drive for overflow files, and the other for backup. This is the best option if you're nervous about installing internal drives, or if your Windows 8 tablet won't allow you to upgrade them.
For better speed, I strongly recommend a USB 3.0 connection for the overflow drive. Also, make sure the backup drive is large enough to back up the data on both your internal and external drives.