Hey I also had this issue and this reddit thread:(https://www.reddit.com/r/MacOSBeta/comments/1dfo2sl/ridiculously_high_disk_write_rate_from_unknown/)
seems to have found the solution. Somehow Spotlight is indexing mass amounts of data when the devise is asleep, so simply turning off indexing and removing reseting the index seems to solve it.
ianalmi
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