My laptop had an M2 SSD and a SATA HDD. I bought a SATA SSD to replace the SSD. I copied everything from the HDD to an external USB drive, then physically replaced the HDD with the new SSD and formatted it using Manage-> Disk Management. When I copied the files I had backed up to the external USB drive back to the new SSD drive, the file copy stared off fine, but then unexpectedly slowed down after a few tens of seconds. After running at the slow speed for about a minute, it sped up again to full speed. This cycle repeated until the file transfer was finished. At first I thought it was the USB HDD, but then I tried with the M2 SSD. This is blazingly fast, so the problem occurs after a significant amount of data has been transferred, but it is there. Initially, the disk usage is low and the transfer speed fast, limited by the slowest disk/ interface. Then suddenly the disk usage on the internal destination disk will go to 100%, regardless of which drive it is. This causes the transfer speed to slow down to about 10 MB/s. After a while, the disk usage automatically drops to a low value and the transfer speed goes back to the previous fast speed. Speeds switch between two values: (ignoring the initial burst of cached speed). USB drive to SATA SSD: ~110 MB/s and ~10 MB/s (USB drive is HDD, so 110MB/s is good) M2 SSD to SATA SSD: ~500 MB/s and ~10MB/s USB drive to M2 SSD: ~110 MB/s and ~10 MB/s SATA SSD to M2 SSD: ~500 MB/s and ~10MB/s The new SSD in question is a Pioneer 1 TB 2.5" (APS-SL3N-1T).