I believe it’s partially due to the SSD architecture it uses. Extreme 32GB flash drive works like a SSD without TRIM (while most conventional USB flash drives do not use so complex controller logic like this). When new, all blocks are considered free so writing speed is high. However, after writing, all written blocks are considered containing data, leaving less and less free blocks. When 32GB data has been written accumulately, no matter how much file data remains on the drive, there will be no free blocks left. There will be a small percentage of blocks used as overprovisioning, say 7% for example. After the overprovisioning space is used up, further writing will meet blocks containing existing data. Thanks to the existence of the overprovisioning, those blocks may be partially full (for example, I have a file A, and I overwrite part of it, then the data overwritten may be saved into further free or partially free blocks, possibly overprovisioning space, and the original blocks containing data of A become partially free). Then, when write goes on, those partially free blocks will be merged with new data to make a full block. But since flash memory writes use blocks as a unit, there will be a “read block, merge data, write block” cycle, making the performance poor, such as the 5~8MB/s as seen (in my own case, its 3~7MB/s ). That’s my explanation to the performance issue.
However, I wish that there is some software way to “reset” the drive, even clearing all the data, this is still acceptable for me. I’m contacting Sandisk support now.