Windows.old is probably one of the worst things that has come out of the dusty corridors of Redmond. Not that the idea itself of Windows.old is bad. Just its implementation.
The origins of Windows.old
Microsoft first introduced the Windows.old feature way back in Windows Vista, the version of Windows that almost everyone hated. The basic idea behind Windows.old, a hidden folder that is automatically created when you upgrade one version of Windows to a more recent version, is that it contains all of the operating system files and personal data files of your previous version of Windows. That way, if you decide after the upgrade that you don't like the new version of Windows, you can use Windows.old to go back to your previous installation of Windows.
The problem with this is that users actually started wanting to use Windows.old to restore their systems to its pre-upgrade state. Microsoft naturally had to oblige by publishing a Knowledge Base article that outlined a step-by-step procedure for restoring a PC to its previous Windows installation after the user installed Windows Vista. Unfortunately, the procedure for using Windows.old to restore an upgraded PC to Windows XP or Windows 2000 turned out to be long and difficult with numerous manual steps the user was required to perform. This meant that the potential for something going wrong during the recovery process was high, which resulted in numerous complaints from frustrated users.
Who needs a backup?
There was an even bigger problem, however, with Windows.old. Since this hidden folder also included a copy of all of the user's own personal files and folders (to ensure everything could be recovered properly in the event of a dis-upgrade) many users thought that Windows.old "automatically backs up" all their data files before upgrading their PCs. In a way, this is true since the Windows.old folder does contain a copy of all of the user's data. But Windows.old is not a backup in the normal sense of the word. It doesn't create a .bak file you can mount with a backup program and use to restore individual volumes, folders, or files. Instead, you have to perform another bunch of steps to extract some or all of your personal files from the Windows.old folder after an upgrade or custom installation of the new version of Windows on your PC. And as this post on SevenForums indicates there's a lot that can go wrong when the average user tries to walk through this procedure.
As we know from the worlds of economics and politics, unintended consequences can often be nasty in the effects they result in. An unintended consequence of the introduction of Windows.old as a feature was that users started to think that they no longer needed to perform a full backup of their PCs before upgrading them to a newer version of Windows. The nasty result of introducing this feature was that a not insignificant number of users lost their personal data files after upgrading their PCs from Windows XP to Windows Vista--or to Windows 7 or Windows 8 or Windows 8.1 or Windows 10 since Windows.old has continued being implemented in these operating systems.
Even the experts seem to have gotten fooled by this one. For example, this PCWorld article from a few years ago suggests copying your Windows.old folder to an external hard drive or burning it to DVD media "just in case" you discover later on that you "need a file or two from it." While the author doesn't state this explicitly, the implication seems to be that the author is suggesting that you don't need a backup of the data from your PC before you upgraded it since you can always grab the files you need from Windows.old.
In my opinion, this sends a bad message for users to take away. Best practice, then, is that you should always make a full backup of a PC before you attempt to upgrade it to a newer version of Windows. Always, always, always. And make sure you verify the backup after creating it to ensure its integrity. Try also restoring a few files and folders from it to make sure you can do this. And make a copy of your backup and store it safely offline somewhere (and not just in the cloud). Then--and only then--should you go ahead and start the upgrade.
What's wrong with Windows.old in Windows 10
So has Microsoft finally ironed out all the bugs now that Windows 10 has arrived? Far from it, unfortunately. The initial release of Windows 10 included the same 30 day rollback period that Windows 8 included. After those 30 days were up, your Windows.old folder would be automatically deleted along with a couple of other hidden files that allowed you to roll back your upgraded PC to its earlier version of Windows. This rollback period stayed the same when the November Update (v.1511) was released last year, but now with the Anniversary Update (v.1607) the rollback period has been inexplicably shrunk to a mere 10 days.
This, in my opinion, is a bad move that was likely adopted by Microsoft because of telemetry gathered from millions of Windows 10 installations. Unfortunately, nowadays, telemetry has become an excuse for not using your brain to actually think about possible scenarios and their outcomes. Sure, the vast majority of users probably decide whether or not to roll back an upgrade within the first couple of days after it has been performed. But what about the 0.001 percent who don't discover a "problem" with the upgrade until a few weeks later?
In fact, instead of having Windows automatically remove the Windows.old folder after 10 (or 30) days, why not instead simply have Windows surface a dialog box saying, "Windows can remove your Windows.old folder to reclaim more disk space you can use. Doing so, however ,will prevent you from being able to roll back your PC to the previous version of Windows that was installed. Do you want to continue? Yes or No."? In other words, stop making "artificially intelligent" decisions on behalf of Windows users and instead give Windows back to its users!
Another problem with Windows.old is that when you update a version of Windows 10 to a newer version of Windows 10, the existing Windows.old folder is replaced with a newer Windows.old based on the newer version of Windows 10. But the new Windows.old no longer contains the files that the old Windows.old contained, so the result can be loss of personal data for the user.
Still another problem is the simple fact that Windows 10 creates a Windows.old folder after each updated version of the operating system is installed even if you don't actually need or want to be able to roll back to the earlier version of Windows. Depending on the amount of personal data on your PC, this Windows.old folder can be huge--multiple dozens of gigabytes are common. For users whose PCs have solid state drives (SSDs) with limited storage space, this can even cause the update process to fail. I've heard of a number of instances where the Windows 10 November Update was unable to update to the Anniversary Update because there wasn't enough free disk space left to create the Windows.old folder and complete the update process. The simple answer for Microsoft is to create another option somewhere in Settings that enables users to turn off Windows.old functionality on their PCs for applying future Windows 10 updates.
The bottom line?
What's the bottom line in all this? Just make sure you fully back up your PC before upgrading it to any new version of Windows including each new release of Windows 10. If you like you can utilize the built-in backup features included in Windows 10 for this purpose. Personally, however, I would recommend using a reliable third-party backup product like Acronis, Macrium Reflect, or EaseUS ToDo before you begin. Consider also using the 3-2-1 rule for your personal data which involves having three copies of your data stored on two different devices plus one additional copy stored offsite. After all, as Dilbert once suggested, you can never have enough backups!
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