I've never been able to successfully use the sync function in MC to go directly from Windows to Android, instead I sync to a local folder on the same hard drive as the library. That way data is only copied from one place on the disc to another and it is fast and works reliably. I then use Resilio Sync to syncronize the Android player with the local copy. I can't say I'm thrilled with Resilio, but it works.
My issue right now is that I'm running out of storage - my PC has a 1TB SSD, and my android player has a 1TB SD card, but with needing two copies of the synced material on the hard drive means I'm limited to half of whatever storage is left after all my other files.
A quick fix would be to just get another SD card, and use that to hold the local copy. Assuming that works. The file copy would be entirely within windows on the same PC, but it would be moving from one physical media to another and I'm not sure where the bottleneck is that makes the direct sync to Android fail - different OS or different physical media?
Wondering if anyone else has tried a similar approach? Comments appreciated. Thanks.