Thank you for the video. By watching what you are doing in the video I was able to reproduce the problem and Kirk and I discussed what was going on. First, what you are seeing is not limited to Collections, but rather any Contact Sheet opened by the Catalog system (Search, Browse, etc.). So here's what's going on and our suggestions:
At the bottom of your Contact Sheet, it lets you know how many items are selected and how much disk space that selection occupies. When you do a Command-A to select all items and the metadata for all items has not yet been loaded, It will say "Size info not available". When it says this, the drag and drop is not going to work, as the operation requires that the metadata has been loaded before it will work.
So there are two workarounds: My suggestion would be to set Lightroom as your Editor in Photo Mechanic's
Launching Preferences and use the Edit shortcut (Command+E on MacOS). This worked well for me even without the Metadata Preload completed. I tested with Lightroom CC.
If you much prefer Drag and Drop, you can force the pre-load to happen by scrolling down to the bottom of the Contact Sheet. Once the file names turn from red to white, you should then be able to select all and then use the drag and drop operation.
Hope that helps!
