USB write speed irregular

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http://askubuntu.com – I would like some advice on why this is happening to my USB devices... I have tried a few USB 2.0 pendrives, FAT32, EXT4, NTFS. All no more than 16 GB big. Read speeds are fine (25-27MB/s) Write speeds however... Start really fast i.e. 50 to 60 MB/s then drops significantly down to 10MB/s before gradually slowing down to 2MB/s and less. Overall the copy process is extremely slow. 30min for 3 files totalling 4.5GB and even slower for 8,422 php/css/html files totalling 352MB. Where do I begin? Google and forums yielded no help. (HowTos)