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Backup, Clone, or Copy? Disk Cloning vs. Disk Imaging vs. Copying
There are several disk backup concepts that are related and the terms are sometimes used interchangeably; however, there are major differences in how and why these backups are done.

What is Disk Backup, a.k.a. Disk Imaging?
When you back up a disk, you create a disk image file that contains the contents of the physical disk you are backing up. The backup software reads the disk and creates a disk image file with the disk contents in it. The advantage here is that you have a file that is likely going to be smaller than the actual disk and you can copy it elsewhere later and archive it. Disadvantage is usually, but not always, that most software uses proprietary formats for these disk image files, but some backup solutions also offer open standard formats for disk images, such as BackupChain.
Another disadvantage here is that the entire disk needs to be scanned in order to create a disk image or an incremental disk image of it.

What is Disk Copying?
Disk copying usually means you want to copy an entire disk to another physical disk at the sector level. Note that would create an exact clone and the problem with that idea is that operating systems do not like to have disk clones connected simultaneously because that would confuse the system. Each disk must have a unique ID; otherwise, the operating system cannot work with it. Ergo, exact disk clones can only be created by a disk duplication system or by keeping the disks always offline, i.e. unmounted.

What is Live Disk Cloning?
Live disk cloning is a technology that allows you to clone a disk to another physical disk while the operating system and all applications are in use. The disk clone can be mounted side-by-side with the original disk, because it has its own unique ID, and is hence because of that not an exact clone; however, the cloned disk can be used to boot Windows and is otherwise identical to the original.

When to Choose Which?
A good Windows Server 2016 backup solution should offer all of the above options, including file-level backup. Disk cloning should be considered for servers where downtime is an issue. Disk image backups, on the other hand, need time to be restored to a new disk. When you have a clone ready to go, all that needs to be done is a change in the BIOS boot settings and the system is ready to reboot.
For data disks, a disk clone is immediately accessible and can also serve as a time-delayed mirror, because it isn't mirroring the data instantaneously like a RAID mirror array. Disk cloning requires that the target disk is always available, online, and physically connected to the server (exception iSCSI and other attached remote storage), whereas disk images can be stored anywhere on other disks or arrays or NAS servers, for example.

And finally a copied disk has the advantage that its contents are immediately available in a way that is native to the operating system, unlike disk images that either require a restore or specialized (usually proprietary) software to access files and folders.

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