The writes are bad but where did the reads go?!

Running a 100MB test would normally be on the small side and as the Samsungs have a cache that is larger you should at least do 500MB.

I'd let it idle for a day or so, it might rely on background GC although a SE should normally take care of all such anomalies.
Could you try running HDTune Read Benchmark using 64KB and 4MB?