Fraps Frame Drop After One Minute of Recording

So I'm trying to record some Overwatch, at 30 frames, and I can get a constant 60 frames while doing so for about a minute before it drops to 19-29. I'm recording to an external drive, and it helped when I started doing that, but then stopped working after one recording. Any help would be appreciated, thank you.

CPU: i5 6300HQ
GPU: NVIDIA GeForce GTX 960M
RAM: 8GB
 
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