I pop this up every so often, but here's some Media Management workarounds that function for the way we use Resolve:
1) limit your session to just the files actually used in the session (that is, make sure no unnecessary files are sitting in bins)
2) Render-in-Place all H.264, JPG, TIFF, and PNG graphics clips to ProRes or DNxHR so that now the clips have embedded timecode and (preferably) unique file names
3) avoid 44.1kHz audio source files (especially MP3) -- convert them in advance to 48kHz / 24-bit WAV files instead
4) for camera clips with embedded audio, my opinion is you're better off if you strip the sound out as a WAV file that lives in the session
5) be aware that Titles can be a bit dodgy and don't always survive the changeover with Media Management. (I would say the same thing with Fusion sequences, which I would render out and treat as a separate transcoded element.)
6) high frame rate (slo-mo) footage can be a problem because the timecode isn't predictable or easily trimmable. Handle those as VFX shots and render them in place first.
7) be wary of PROXY FILES cluttering up your timeline. Make sure that all the files are legitimate camera raw files (assuming that's what you want), because otherwise that's a guaranteed fail.
The simpler you make your session, the better the potential for successful Media Management. The moment you have a filename clash or a timecode conflict, it can fail. I wish Resolve had better error trapping so that when it did encounter an error, it just popped up a message with a list of problems, rather than just bailing on the Media Management process entirely.
If your file copying is failing for another reason, try Nikolai Waldman's Resolve Collect and I bet it'll get you at least 98% there without errors. His program has been a lifesaver for me over the last 6-7 years.
http://www.niwa.nu/resolve-collect/Another possibility (which I haven't tried) is EditSpy for Windows:
http://edlspy.felixhuesken.de/