Hi @Андрей Михалевский,
To me it is the expected behavior of getting the error StalledDueToTarget_DiskLatency during worktime, as migration has lower priority than mail flow and client access.
This is designed to ensure user experience is not affected.
If possible please perform the migration during off-hours so it can use more resource.
If you would like to check if you are having issues with the server disk, when you are seeing some performance issues, please have a check of the following:
1.in application log check if there are event 509 and 118
2.in performance monitor add the counts to check the database read/write Average Latency
Refer to this link: Database Counters
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