snolly

So, firmware M3CR046 that solves the issue is not available from Crucial in a direct download. You need to use Windows and Crucial Executive software which is crappy and some people are mentioning that in order to update the firmware this way you need to be able to write to it and actually have something being written to the disk while updating otherwise the process will fail - which is totally ridiculous. So I made a bootable portable Win11 usb stick which took 3 hours to do and the Crucial software does not launch in there (crashes).
 
I then combed the interwebs and I found a user who contacted Crucial and they gave him the firmware file. Thread here and I also attached the FW on this post. You need to unzip the zip file and use the .bin file in there.
 
I had a BTRFS cache with 2 affected MX500 drives in there.
 
Stopped the array Removed one if the MX500. Started the array and waited for it to rebuild the BTRFS cache. Now one of the MX500 was listed as unassigned device. Copied the FW .bin file somewhere in Unraid run this command  hdparm --fwdownload /mnt/user/athens/temp/mx500/1.bin --yes-i-know-what-i-am-doing --please-destroy-my-drive /dev/sdb where first argument is the path to the .bin file and the second one is the name of the drive I need to update. In this case /dev/sdb rebooted the server and I checked that the drive has been updated to M3CR046 (click on the drive's name and go to identity tab) stopped the array and added the updated drive back to the cache. started the array and wait for the cache rebuilt. stopped the array removed the other drive and then do steps 3 to 9 for the second drive. I hope this helps anyone who faces the same issue. Hopefully I will not face the same problems again.
M3CR046.zip