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Partitions are a way of dividing up a hard drive's free space. Each partition will appear as a different physical disk in Windows Explorer. A partition can be formatted without risk to any other partitions on the big hard drive.
Multiple smaller partitions will still perform as a single hard drive, so there is no advantage to placing the swap file on a separate partition on the same hard drive. This is also not a good strategy for backups. While some malicious software may only tamper with the data on one of your partitions and leave a backup partition alone, a hardware failure will damage all partitions on your hard drive.
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Easily Taking Advantage of All the Free Space
The main reason a big hard drive might be best as one large partition is that you can use the entire free space of your big hard drive with no fuss. If you divide it into smaller partitions and one of those smaller partitions fills up, you will have to resize the partitions, which carries some risk of data loss, and is also harder if it's the operating system's partition that needs to be resized.
For example, let's say you set up half the big hard drive for Windows system files and your installed programs, and half for your personal data. Later, you fill up your personal data partition, but the large hard drive still has a good amount of free space on the Windows partition. This will require either mixing your data into the Windows partition or resizing the partitions. If you want to resize your Windows partition, you can't do that while Windows is running. Of course, if you set up your big hard drive as one large partition, taking advantage of free space is not an issue.
Image Source: Wikimedia Commons/Zzubnik
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Separating your Personal Data from the Operating System's Files
Having separate partitions for Windows and for your data files makes the process of reinstalling Windows without losing your data much easier and cleaner. The Windows installation process wipes the Windows system partition completely, but your data will still be untouched on the other partition.
The same goes for installing new operating systems; whether upgrading to a new version of Windows or switching to Linux, the operating system install process preserves your data. With a single large partition, this process would require first backing up your data to a different hard drive, USB stick, or recordable DVD or CD, then restoring your personal data after the new operating system is installed. With smaller partitions, your personal data is immediately good to go.
Some people also consider smaller partitions with designated purposes to be very helpful for organizing data.