HUGE FPS Drop when recording?

Hey guys, i have HUGE fps drops when recording in battle field 3, i run on high/ultra, at about 70-80 fps average, but when recording i go down to about 40-50 fps, even at places where i have about 150 fps! Also my GPU Usage goes from about 80% on both cards to 30-40%. I Record to an external hard drive, which has worked extremely well before, but for some reason the fps drop dramatically now, theres enough free space on the HDD. Thanks for any help!

My specs if their needed:
AMD Phenom II X6 1100T @ 4GHz
Asus Crosshair V Formula
8GB Corsair XMS3 1333MHz Ram
2x Radeon HD 6950 in CrossfireX
PSU: OCZ ZX1250w 80+ Gold

I use MSI Afterburner for recording.