I have a USB 3.0 External HDD, and it just randomly started to record at around 50 fps instead of 60.
This is what it looks like: https://www.youtube.com/watch?v=myNZud149Yc
This is what the recordings look like (But usually longer): https://youtu.be/zojZXNLGdHg
This is the HDD: http://www.amazon.com/gp/product/B00TKFEE5S?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s00
My motherboard is an ASUS H170 Gaming Pro 1151 Socket, with an i7 6700 CPU.
Just as you said. The HDD writing speed might be too slow for using lossless recording (I'm assuming you are using Lagarith, UTvideo or MagicYUV) on 1080p60fps. I'm surprised it even managed to hit 60fps at that resolution. On a fast-paced game, there's a lot more data transfer from lossless and a slow HDD will bottleneck the recording FPS. Either push the resolution down to 720, or push the fps to 30fps and give it another shot.
Btw what codec are you use?
For fullhd 60fps lossless you need like 118MB/s, your hdd can write up to 140 MB/s max thru usb 3.0, but due windows usb drivers (windows usb3 drivers are slower than producent drivers, on windows 7 sometimes even 40%) it usually write like 98Mb/s and it is not enough for 60fps. And in your video you show that you have system/usb3/disk problem what is not realted to dxtory. I would try format usb disk again something like ntfs with 64k cluster and would try enable advanced cache for this disk in device manager, cause all usb disks are default set for fast remove