Stupid “Assembly-CSharp-Editor.dll.mdb is denied” error has not been fixed. Here I was hoping that Unity 5 would FINALLY allow me to have my real-time virus scanner running while working in Unity. Nope.
BTW, this is (or was) in the known issues thread so don’t tell me to report it. I did that a long time ago!
What anti-virus software are you using? The issue is not with Unity but with the virus scanner’s protection settings. Can’t you set the dll.mdb file (or the entire project folder if you so desire) as an exception to your anti-virus’ ruleset?
I’ve never had this occur with Microsoft Security Essentials.
I use MacAfee (because it comes with my internet service) and there is no way that I have found to do that. Only option I have discovered has been to turn off real time scanning.
You could simply add the two files to McAfee’s exceptions list using this guide.
http://forum.unity3d.com/threads/mcafee-issue-solved.288885/
I did try that when I first started having this issue a couple of years ago. Unfortunately, my version doesn’t seem to have an option to exclude specific files or folders. I just get a bunch of check boxes like “scan all files” or “scan only programs and downloads”. I suspect because it’s the "Cox Security Suite Powered by MacAfee I don’t have all the available options.
Honestly, I just wanted to complain. If I were looking for help, I would have posted in the proper forum. 
On Windows I’ve avoided that error by selecting “Run as Administrator” when I start Unity.
Norton doesn’t complain. It does delay webplayer loading until its finished scanning everything. Bit of a pain. Especially since its scanning the same file each time. Either way its a virus checker issue and not a Unity problem
I totally understand this. That’s why I wrote a page long post describing how I built a configurable input system for the exact same reason. Lets curse the things developers haven’t got to yet to fixing!
Die Unity Terrain.
Welcome to the free edition of normally paid anti-virus suites. This is why I love Microsoft Security Essentials. It doesn’t have a paid version to annoy me into buying. 
Oh, it’s not free. It’s part of my internet package so I pay for it that way. It’s supposed to be full-featured.
It’s still a giveaway. I’ve used it when I had Cox, before I switched over to centurylink. Most ISPs offer some form of virus scan but it isn’t full featured. It does the same job scanning but it’s not as customizable.
That certainly seems to be the case here. Anyway, I just meant it’s somewhat better than most “free” virus scanners.
Sometimes, it is a better deal to not use every single feature something comes with. I’m paying for a package with cable, internet, and phone. I don’t use the phone, but with the package deals, I’m paying $10 less by having it than without it. And I literally don’t even have a home phone device, as we have gotten used to cell phones completely and haven’t even had a home phone in years.
So, if you can get a free anti-virus that works better than that one, you are better off using it instead. I for one have never like McAfee and Norton products, mainly because I can get free software that works just as well, maybe better, and typically less weight without all the extra stuff too. This may simply be because most of us here know how to work with computers and what to avoid(etc…) so we don’t need some of the stuff that comes with the expensive paid packages.
I use Avast, and have done so for years. I have NEVER had any issues with it in Unity projects. At some points, it has actually interfered with running GMStudio(and older GameMaker versions) projects, but I think that is simply due to the nature of those executables, but it never bothered me that much because it is easy enough to turn off the full time scanning.
This happened to me too, after my McAfee trial ended. I had to uninstall it and turn on my firewall and it Unity stop giving errors to it.
When I had McAfee I had to turn the Real Time Scanning off whenever I went to use Unity.
I was mostly referring to the free copies of Norton included with OEMs.