I've seen some flash drives that prepare a pool of free AU blocks which is common with Phison controllers They prepare and erase a new pool only when it's full

I've seen some flash drives that prepare a pool of free AU blocks which is common with Phison controllers They prepare and erase a new pool only when it's full causing bursts in speeds and long periods of unresponsiveness (10 sec) while writing, reading is blocked as well. You can see it as a zigzag in file transfer speed graphs. This is not to be confused with OS buffer flushing. While I couldn't find any official info for the reasons behind it, it's assumed that the erase speed is slow on low quality flash and hence the controller has to pool in order to erase pages in parallel, increasing speed when copying small files between computers on plug and unplug. In my case it was about 500MB before before freezing on a 128GB flash drive.