File copy speed drops to zero and stays there mid-transfer

A zero-speed display does not distinguish a slow operation from an I/O failure, and a clean filesystem check does not identify every possible storage problem. Since you already tested cables and ports, preserve the original files and capture a controlled transfer result. For one test file, run robocopy "SOURCE" "NEW-DESTINATION" "FILENAME" /J /R:1 /W:2 /LOG:copy-test.log, replacing the placeholders. This uses unbuffered copying, limits retries, and records errors. Do not use /MOVE, /MIR, or /PURGE for this test. Compare the same file in the opposite direction, then with another destination if available. Check Event Viewer around the stall for disk, filesystem, or storage-controller errors. Re-enable antivirus, since disabling it did not change the behavior. If the transfer reports an error, share its exact code, direction, drive models, and connection type; that is stronger evidence than assuming the progress bar proves a particular cause.