I have a customer with MX400's and it was a couple years ago, but I recall they reported a beeping sound from one and it turned out a PSU had failed. We simply got an RMA through support and the end customer swapped the power supply himself. I am a little unclear exactly what you are asking but yes, the power supplies are modular and can be swapped. I don't recall if it was a hot swap or if support advised to shut down first though. This reminds me of the fact that there was no event log or email alert for the PSU failure event. We would have never known if someone had not heard it beeping and investigated.
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