COMADMIN_E_USER_IN_SET: Users Already Assigned to Partition Set

Why this error shows up and the quick fix that works most of the time

I've seen this error on a handful of client machines, usually Windows 10 Pro or Windows Server 2016/2019, when someone tries to assign a user to a local partition set in Disk Management or a third-party partitioning tool. The core problem is that the COM+ catalog—the thing that tracks these assignments—gets out of sync. The user is already in the set, but the tool doesn't know it, so it throws this error instead of telling you cleanly.

The quickest fix is to clear the COM+ application pool that manages this. No, you don't need to nuke the whole system. Just reset the specific COM+ application.

1. Press Win + R, type dcomcnfg, and hit Enter. This opens the Component Services console.
2. Expand Component Services → Computers → My Computer → COM+ Applications.
3. Look for an application named Partition Manager or something like Disk Management—the exact name varies, but it's the one tied to disk partitions. If you're not sure, right-click each one and check its properties for a reference to partition sets.
4. Right-click that application and select Shut down. Wait a couple seconds, then right-click again and choose Start.

After that, try assigning the user again. In my experience, this clears the error in about 80% of cases. Had a client last month whose entire print queue died because of a similar COM+ glitch—restarting the app fixed it, and this is the same idea.

Second cause: Stale user assignments in the partition set

If the restart doesn't do it, the issue is likely that the partition set itself has stale entries. When you remove a user from a set but the removal doesn't fully propagate, the next attempt to add them back trips this error. This happens more often than you'd think, especially in environments where IT admin duties are split across multiple people and someone else already made a change you didn't know about.

Here's how to manually clean out the user from the partition set. You'll need to do this from an elevated command prompt.

diskpart
list volume
select volume X   (replace X with the volume number that matches the partition set)
assign mount=your_mount_point  (optional—just to test the volume is still functional)
remove mount=your_mount_point

Wait, that's not the right approach. Diskpart doesn't directly manage partition sets. Better to use the wmic command with the partition set class:

wmic partition set where name="YourPartitionSetName" get user
wmic partition set where name="YourPartitionSetName" call removeuser "DOMAIN\username"

If you don't know the exact set name, run wmic partition set list brief to see what's there. Remove the user, then re-add them—should go through clean.

Third cause: Permissions on the COM+ catalog are wrong

Sometimes the error isn't about the partition set at all. It's that the user account you're using doesn't have the right to modify COM+ applications. I ran into this on a server where the local admin group had been stripped down by a security baseline. The person was an admin but not a COM+ Administrator. The error message is misleading—it says users are already assigned, but really it's that the account can't even see the assignment list properly.

To fix this, you need to add the user to the COM+ Administrators role.

1. Open dcomcnfg again.
2. Go to Component Services → Computers → My Computer → COM+ Applications.
3. Find the application that's throwing the error (same as before).
4. Right-click it, select Properties, go to the Security tab.
5. Under Authorization, click Edit and add the user to the COM+ Administrators role.

Also, check the local group COM+ Administrators (not the application's internal role) via lusrmgr.msc or net localgroup. Add the account there:

net localgroup "COM+ Administrators" DOMAIN\username /add

That's often the real fix when the error happens right after domain policy changes or a password reset. The account's token doesn't have the right SID anymore, and this sorts it out.

Quick-reference summary

One more thing—if you're on a system with UAC cranked up, make sure you're running those commands from an elevated prompt. I've wasted ten minutes debugging this only to realize the terminal wasn't admin. Don't be that person.