New 1803 version created a new disk partition - Can I reclaim it?

Yes, using the DISKPART commands will clear up the issue. FWIW, I went straight to MiniTool Partition Wizard (Free) when diskmgmt.msc came up short, and its right-click menu does the job just fine if you assign the offending partition a drive letter = "None". I'll blog about this issue and its frequent appearance tomorrow for Windows Enterprise Desktop. So far, it's showed up on only 1 of the 4 machines I've upgrade (1 more left to go, though).
HTH,
--Ed--

ikjadoon said:

What's actually happened:

1803 added a driver letter to the partition. Slicendice is correct that these partitions have always existed, tucked away neatly in Disk Management, but for some reason, 1803 is adding a driver letter:



Now it does...for no reason. Many users have reported this here. And here. And here. Besides the un-needed drive letter, it's also been given storage warnings. So for me, I get these every ~30 minutes:



I imagine this issue will be more heavily reported on TenForums in the coming days and weeks, unless MS figures out what went wrong.

What's the recommended fix here? I think removing the drive letter via DISKPART (as the first link ^^ shows) will solve both the File Explorer appearance and the storage warnings, right?

Yep, I have seen those reports too.

Safest thing to do is to just un-assign the drive letter.

This is how my EFI/GPT based system look like, which is following Windows recommendations:


The MS "System Reserved" is missing for some reason, but normally that is also included. My recovery partition is also empty, but I could put WinRE or another custom recovery image there for easy repair.

It's an excellent tool: I blogged about it for Win10.Guru on April 17: Toolkit Item: MiniTool Partition Wizards (MTPW). Worth installing and getting to know.
--Ed--

NavyLCDR said:

Now what you can do with MiniTool Partition Wizard is move the FAT32 100 MB partition to the front of the drive. Then delete the 16MB (Other) partition. I've never had luck moving it. That is the Microsoft System Reserved (MSR) partition. If you want to recreate - make sure to apply the changes in MiniTool Partition Wizard and then exit it. Run the diskpart command. Then recreate the MSR partition using the command:
select disk 0
create partition MSR size=16
exit
exit

Then you can go back into MiniTool Partition Wizard and move the front of C: drive to add the unallocated space to C: drive.

NOTE: Disk 1 in MiniTool Partition Wizard is Disk 0 in diskpart!

Code:

Microsoft Windows [Version 10.0.17134.1]
(c) 2018 Microsoft Corporation. All rights reserved.

C:\WINDOWS\system32>diskpart

Microsoft DiskPart version 10.0.17134.1

Copyright (C) Microsoft Corporation.
On computer: MAIN-DESKTOP

DISKPART> select disk 0

Disk 0 is now the selected disk.

DISKPART> create partition MSR size=16

DiskPart succeeded in creating the specified partition.

DISKPART> exit

Leaving DiskPart...

C:\WINDOWS\system32>exit

You can move the MSR partition using Macrium Reflect. You basically make an image backup, then restore backup using drag and drop so the EFI, MSR and C drive are all next to each other leaving the unallocated space to the right of the C drive.

However, a lot of work for very little gain.

The MSR partition is a mystery, and it is not even needed for bitlocker in fact.