I bought two SanDisk Ultra USB 3.0 Flash Drive 256GB a few weeks ago from SanDisk via Amazon.
They were formatted as fat32 when they arrived so i reformatted them both as exfat because i am storing large iso files on them. When I try to copy a 4GB iso file to both of the drives it starts at 62MB/s then drops to 5MB/s then drops to a crawl 350KB/s and says it will take 24 hours to finish. I have changed the usb device properties to better performance and i have tried them in usb2 slots. I have also tried them on various machines i have ranging from old machines to brand new machines without any change. I have also tried ntfs with the same problem. I have also tried a few small files and they transfer fast, but a lot of small files and again a crawl.
Any ideas before i return them both?