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I am using a 1TB SSD drive for file transfer between two Windows 10 computers that i connect using a USB 3 adapter and ports. I copy files from System A to the drive and then from the drive onto System B. I've performed one transfer from A to SSD to B at relatively constant rate of 180 MB/s. Once i've emptied the drive onto System B i needed to perform another transfer from System A to the drive and the speed of writing from A to SSD started to fluctuate similar to this: USB random write speed

If the first transfer was fine then it is not a hardware limitation. Could there be a process in Windows that affects cache on an SSD/USB drive ?

One thing to note is when plugging the SSD thorough USB into System A for the second transfer it did show a second 0.5 GB drive called System Recovery that i did not create. Could this be a symptom of what could be causing it?

If not, is there a way to flush the SSD cache ?

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You ignored two possible bottlenecks:

  1. You successfully kept the type of drive of your system A a secret. If the drive is a hard disk drive you might deal with defragmentation and files of varying size in system A which could heavily affect transfer rates.

  2. If the first transfer was fine then it is not a hardware limitation.

Wrong. Once your SSD is filled up the write process on the SSD will slow down due to the necessary time-expensive erase functions. Your SSD firmware cannot react as fast in erasing as the operating system sends TRIM commands to the drive when you are deleting the content of your transfer SSD to be able to copy another round of data on it.

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  • I don't think that TRIM is related to this problem. More likely is that the SSD contains an small SLC cache part that can be written fast, but once the SLC cache is full the remaining TLC cells have to be used and the SLC cache needs to be flushed into TLC cells and this causes the speed degrade.
    – Robert
    Commented Nov 10, 2021 at 11:16
  • Robert, VilleLiponen said: "I've performed one transfer from A to SSD to B at relatively constant rate of 180 MB/s." Your explanation only fits if the SLC cache is as big as the whole SSD. Otherwise speed variations would have occured already during the copy process before the SSD was filled up completely. His problems started on the second fill, though.
    – r2d3
    Commented Nov 10, 2021 at 14:24
  • I don't see where in the question is mentioned that the first copy process consisted of 1TB files. I can only read "I copy files from System A to the drive" but the size is never mentioned. Could be 1GB or 5Gb or 10GP or the whole drive as you have assumed.
    – Robert
    Commented Nov 10, 2021 at 14:31
  • I don't understand your point. I did not say anything relating to a file size of 1TB. As for your statement: "I don't see where in the question is mentioned that the first copy process consisted of 1TB files." I don't see that either. What are you talking about?!
    – r2d3
    Commented Nov 10, 2021 at 14:41
  • You wrote "Your explanation only fits if the SLC cache is as big as the whole SSD" and this was a comment on the first transfer A to SSD. So If my explanation only fits if the SLC cache is as big as the whole SSD the data transfer must have been the size of the whole SSD, otherwise your comment would not match. Thus you assume that the first transfer was 1TB.
    – Robert
    Commented Nov 10, 2021 at 14:54

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