Why would you download the entire folder of almost 1GB for a few kB's, instead of just getting the files that you needed?
...Click on one file to get one file, tick multiple files and click download to have it archive and download multiple files.
The missing file is now online for everyone.
Because cherry picking through OneDrive is such a pain in the a** to go through because the way they have it set up.
Right now, the most recent date listed under PinMAME roms folder (if I sort by date modified) is 1/14/2016. If you uploaded the one missing rom today, wouldn't it show 1/16/2016 for the most recent entry? Nope, it doesn't. Now I have to figure which set you uploaded.
On top of it, OneDrive can only display 200 files at once. Then, it has to cache the next 200 files to display and so on. It's tedious.