Reviewing this old thread, here's some reminders on Media Management in 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) 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
4) 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.)
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 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/-----------------------------------------------------------------------------
I just survived a very challenging conform / media management project in the last 2 weeks, and I can say that Resolve 18.1.4 now has better handling of H.264, JPG and other graphics formats. We did have some weirdness with some PNGs, but that was a simple fix.