Well, if this is the first time that you've ever written to it, then the firmware automatically has a secondary "read verify" enabled for those sectors. It's a way of double-checking new drives for any defects. If you run HDTach 2.61's write-test (Warning: destructive!), then you can see the slow write-speed scores. If you interrupt it in the middle, and then start it over again, it will go at twice the speed through the sectors it has already gone through, and then slow down again once it hits the area of formerly-unwritten sectors.
Also, doing a regular NTFS format rather than a quick NTFS format, will do a read-verify surface-scanning pass over the disk volume as well, and for an entire 300GB, that will take a while.