On my EX3 is am using a E-Films MxR ExpressCard SxS Replacement Adapter with an Adata class 10 SD card. When I go to 720 30p and crank it to 60 I get my slow mo. I did a 5 minute test at the slow mo setting.... About 3 minutes in I did get a media error in my viewfinder however all the video turned out fine and the slow mo was all there and looked great.
Question is...am I doing any harm by letting the camera continue to roll even though I am getting a 'media error' in the viewfinder.
I see no problem with the footage....it all looked fine.

Did you have new firmware updated on your EX3 ?
Any thing you get from over crank (slow mo) with other medias besides Sony SxS card is bonus (Sony doesn't recommend it). For the media error is a warning message when it detected something wrong with the writing process to the memory card . You should run a good test on your card to make sure it won't happen in other recording, and I don't think it will harm your camera.

Yes I just had the latest firmware update April 2nd. I did run at test at regular speed and get no 'media error'.
Only get it in the slow mo setting but as I said, and it turns out fine. As long as there is no chance of it affecting the camera, I guess I will continue slow mo with the class 10 card when I need it. I am guessing the class 10 card may be fast enough to use for slow mo on occasion.

I use a sandisc ultra II with 30p slo mo and have never had a problem. But I didn't slo mo for that long either. I've never found a reason to slo mo for more than even a minute. Usually much less than that.