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Investigating 1903 PortableBaseLayer Partition


Hmmm. There’s a new, restricted use Windows 10 partition that shows up on some Windows 10 installations. Different sources offer varying explanations for its presence and function. Maddeningly, my own testing is inconclusive, too. This partition is named PortableBaseLayer. Here’s what it looks like on my 1909 Release Preview PC (a Lenovo Yoga X380 Thinkpad):

PortableBaseLayer.diskmgmt

The PortableBaseLayer partition shows up as a separate disk device when I enable the Windows Sandbox. But there’s more to it than that…

What’s Up with the PortableBaseLayer Partition?

Even more maddeningly, MiniTool Partition Manager won’t let me explore the contents of the PortableBaseLayer partition, though it cheerfully reports 4% of its 8.0 GB storage extent as occupied. So I fired the Sandbox up, and used it to download around a dozen apps. This made no change to the storage info for the PortableBaseLayer partition, which stayed fixed at 4%. So, there goes the theory that this has anything to do with Sandbox storage.

One theory about this partition is tied to Reserved Storage for Windows Update. This is presented pretty convincingly in a TenForums tutorial entitled “How to Enable or Disable Reserved Storage for Windows Update in Windows 10.” Basically, it explains that in a future release (still TBD) Windows 10 will use reserved storage for “updates, apps, temporary files and system caches.” The same intro goes onto say that “Microsoft’s goal is to improve the day-to-day function of your PC by ensuring critical OS functions always have access to disk space.” Furthermore, it says that “Reserved storage will be introduced automatically on devices that come with version 1903 pre-installed or those where 1903 was clean installed.”

According to Shawn Brink, author of the tutorial, this feature is available in Windows 10 Build 18298 and higher. He shows the Registry key that controls this behavior in Windows 10 and provides executable .reg files to enable or disable this capability. He also observes that “Your changes will take affect [sic: effect] the next time Windows 10 is updated to a new build.” The registry key is of type Dword. It’s named ShippedWithReserves, and it resides in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\ReserveManager. For what it’s worth, website TheWindowsClub also agrees with Brink’s assessment and attributes its presence to Reserved Storage.

The Windows Sandbox Throws Up Some Grit

The interesting thing is — and I have confirmed this by experiment myself on a couple of my PCs — if you enable Sandbox on 1903 machines, the PortableBaseLayer partition appears. And should you disable Sandbox, it disappears. This would suggest some kind of relationship between the two, though as I have already shown, it doesn’t look like Sandbox uses the partition for storage. That’s probably why a whole slew of websites, including winhelponline, WindowsLoop, Macrium’s forums, and even Quora all assert that the PortableBaseLayer and Sandbox are causally and consequentially connected to one another.

That said, my Fast Ring Preview test machine — an older Yoga X220 Tablet — has Sandbox enabled (and working surprisingly well, considering all the reports I see from other users about slow performance or outright failure on their PCs). It does NOT have the PortableBaseLayer partition in Disk Management. I read somewhere that this mystery partition is unique to 1903 and doesn’t appear in 20H1 preview releases. In at least one case (the X220 Tablet) that is indisputably true. Makes me wonder: what is the real deal for PortableBaseLayer?

A Different Type of Test: New Computer Gets 1903 Upgrade

I ordered an older Lenovo computer last week — another Yoga X380, in fact — that showed up today with 1803 installed. I’m in the process of applying updates, after which I’ll make the registry key change. Once I upgrade to 1903, I’ll also make sure Sandbox is disabled. I’ll create the ReserveManager registry key and set it to enabled (value: 1). Then I can check to see if the PortableBaseLayer shows up, as the TenForums Tutorial cited earlier avers it should. We’ll see, and then maybe we can decide what’s really going on here. I’ve also launched an inquiry with MS Recovery guru Aaron Lower, and hope he might also shed some light on this tricky situation. Stay tuned!

Author: Ed Tittel

Ed Tittel is a 30-plus-year computer industry veteran. He’s a Princeton and multiple University of Texas graduate who’s worked in IT since 1981 when he started his first programming job. Over the past three decades he’s also worked as a manager, technical evangelist, consultant, trainer, and an expert witness. See his professional bio for all the details.

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