Synchronization taking forever

The sync tool will take along time if you're comparing the entirety of two large (as in number of items) drives.

For that kind of job, I'd recommend a dedicated comparison tool like WinMerge (free) or Beyond Compare (better but not free) or similar. Most file/folder comparison tools can be launched via an Opus toolbar button (or hotkey, etc.) against the left and right folders to integrate them really well: