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Someone that understands brushless motors ?

Technically a brushless motor is an AC synchronous motor. Brushless motors can have 8, 12 or even more poles. Here is a cutaway of a 12 pole heli outrunner motor. But then I'm no expert on the subject, just 20 years experience with RC vehicles.
 

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One point to note on WD-40 is it is not a lubricant, it's a degreaser and Water Displacer. It's used to unstick things by dissolving old oils and other contaminants. After using it you need to make sure to add the proper type and amount of lubricant.

It's why if you use it on a door hinge it works great at first, but then you need to keep re-applying it as it's lubricating qualities are minor and it's main purpose is to clean.
 
@cpper I am curious did you try checking whether or not there was anything in the air gap i.e. anything obstructing the slip of paper between the stator poles and the inner face of the rotor?

I would be wary of applying a solvent near or on the bearing. Whilst I might wipe off any 'escaped' grease, but actually applying a solvent to do so is, I think, risky, if it penetrates the bearing it may degrade the grease inside the bearing.
 
My Mini SE have that too: other 3 mother have "clicky" magnetic feeling when turned by hand... but left rear motor doesn't have that.. it flies fine, no abnormal warning, no abnormal sound, all normal...

weeks ago, that left rear motor starts to "stiff" when turned by hand.. after blowing some air from my camera cleaning blower, and arming the motor, problem gone... still that particular motor doesn't have clicky magnetic feel, and fly just fine...
 
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I am experiencing the same issue with my DJI Mini SE Motor. I wondered about the IPA solution and if after some period of inactivity some corrosion might have taken hold. The exact situation you describe has occurred with my drone. I am sending it away for repairs
 
Recently I had a strange issue with my Mini 2, I hope that there is someone here that understands brushless motors and could explain what went wrong. So, this is what happened:

In a beautiful morning, some days ago, I took my Mini 2 out to record some nice holiday footage. I landed the drone in my hand in order to avoid stuff getting into the motors when landing on the ground. All good and no issues. I folded the drone and carefully put it back in the case. Then, in the evening, I wanted to take some more shots, so I took the drone out again and put in on a clean picnic table in order to take off. When starting the motors with the CSC, the left rear motor wouldn't rotate, and the other three would stop after rotating for two seconds. In the DJI Fly app I got an error saying "Motor unable to rotate. Check motor". I tried for a few times, powered the drone off and on again, made sure the props don't touch anything, but with the same result. Then, with the drone powered off, I manually rotated the motor, and noticed it had a different feeling than the other three. The three motors that worked fine all had a similar feeling: they would spin very easily and had a regular magnetic clicking/snapping feel. The bad motor however didn't have this snapping feel, and was more difficult to rotate. It still was rotating very smoothly, but it just required more force to spin it. There was absolutely no grinding feel/sound, confirming the issues was not something getting into the motor.
I was 100% sure nothing got into it, also because of how careful I handle the drone. Regardless, I still removed the props, carefully inspected it using a magnifier, lightly tapped it with some plastic thing, and tried to vacuum out any potential stuff that got into it. All without results, because this was not the issue. The error was still there, and the motor wouldn't start. Disappointed, I folded the drone and put it back in the case.
The next day, I wanted to show the issue to my brother. So I took out and unfolded the drone, and I asked him to manfully rotate the motors to see what I was talking about. He couldn't feel any difference between the four motors, and neither did I. The issues was magically gone, the troubled motor would easily spin and have the snappy feel like the other three. I powered the drone and all motor worked fine, and the drone flew flawlessly.
So, what went wrong here? Is it possible for a bug in the software to cause this issue, and the different "feel" of the motor, when manually rotating it ?
I have had the same thing happen, it is a strange thing that has happened with electronic things, the last thing I was involved with was an AM/FM portable radio, that would not work for it's owner who turned it on and off for me before I touched it - it did not work, I took it and turned it on - it has been working anytime it is turned on since??? TRUE.
 
Something that might help 'clean out the motor area would be some canned air. Use sparingly, not full blast. It would an alternative to vacuuming.
After reading this thread, it does seem like something else is going on, but having the air handy would clear up the 'debris' thought.
Might add, do NOT shake the can of gas or hold in any position except straight up, or it may liquify the gas and squirt into what you are working on.
 
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