iMovie error -49. Can't finalize. Any advice please

Turn off Time Machine.

Just to explain why:

The iMovie files are being written to backup by Time Machine. Due the files being in use by another process, iMovie can't finish the processes that it began.

Turning off Time Machine or allowing several minutes to hours to complete the TM write will cease the error.

There is also a Terminal method to turn off Local Snapshots:

http://reinout.vanrees.org/weblog/2012/02/23/imovie-not-completing.html

I had a similar error and tried all suggestions on the boards.. turned off time machine, checked the RAM, deleted thumbnails in iMovie and nothing worked.

The only thing that worked for me was skipping the "finalize" stage and just going straight to "export" but make sure you export to the "Media Browser". You can then open up the Media Browser from Garage Band or another application and drag your completed video to the folder of your choice. Hope that helps.