Doing a shoot today, during recording I get this message on screen:

"Buffer overflow. Writing to the media was not completed in time."

Camera stops recording, I cannot resume recording, I have to unplug the power and pull the battery to get it going again. Some of my settings get changed when it comes back on (shutter speed), probably White Balance too. Luckily I have another camera running from another angle, and the shot probably won't be needed anyway, but still very concerning.

This happened while recording to a brand new Sony SDXC Class 10 U3 64GB card, in the B slot of the camera, recording using the XAVC codec, 1080p30 @ 50Mbps. I had successfully recorded about an hour to an identical card in slot A before this happened, so I guess the card could be bad? I resumed recording writing to both cards so hopefully if that happens again it won't stop the shot.

Anyone else have this happen?

According to the manual:
"Buffer overflow. Writing to the media was not completed in time.
• You have performed recording and erasing operations repeatedly, or a memory card that has been formatted by other device is used. Copy and take data backup to other device (such as a computer), and then [Empty] the memory card on the camcorder (p. 79)."

I had just formatted both cards on the camera last night so I don't really know what the problem was.

I have had the same message a few times.
Had a few bumps with the camera before this happening.

My solution was to remove the card and put it back in.
No more 'Buffer Overflow' messages.

Very annoying though...

Hope this helps.

What is most concerning is it happened about 20 minutes in, so I wonder is there any way to test the card for a fault before getting into a recording next time?

I did another 1hr + session after this happened recording to both cards with no more problems. Hopefully this was just a one-time issue.

What brand of card are you guys using? This usually means the write speed on your card is too slow.

Sandisk Ultras, even though they guarantee 10MB/s can only sustain 35mbps on the first write. Every successive write will cause recordings to stop. I abandoned Sandisk Ultra after it clearly cannot handle even standard XDCAM EX.

Agree with Jack. It is the card.

As I said before, I'm using these-
"Sony 64GB SDXC High Speed Memory Card (SF64UZ/TQN)"
http://www.amazon.com/gp/product/B00ND1LFPW/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A17MC6HOH9AVE6

If you can't trust a Sony branded high-speed card then what can you trust?

I use a Lexar SDXC 600.

As I said before, I'm using these-
"Sony 64GB SDXC High Speed Memory Card (SF64UZ/TQN)"
Amazon.com: Sony 64GB SDXC High Speed Memory Card (SF64UZ/TQN): Computers & Accessories (http://www.amazon.com/gp/product/B00ND1LFPW/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A17MC6HOH9AVE6)

If you can't trust a Sony branded high-speed card then what can you trust?

I agree.
I am using a pair of these and have had no problem so far.
This is the exact card that Sony specifically recommends for this camera.
I'm guessing that maybe it's just a one-off defective card.
Maybe Sony will replace it...

+1 on Robert's comments. I've been using the same Sony cards for a couple of years on an FS700 with no issues. Now using them on an X70, early days admittedly but so far so good even in 50-mbit XAVC mode.

Try throwing the card back as Robert suggested. I had a similar issue with a Lexar 800 CF card causing dropping bitrates in a Nanoflash. It was the card and Lexar replaced without any issues.

Chris Young
CYV Productions
Sydney