Cant delete Time Machine .inProgress file After over a month of not running Time Machine, I tried to run it again, and it was stuck on "Preparing Backup" for ages (more than 10 hours). 
A Google search has led me to try trashing the .inProgress file. 

Now that file is in the trash, and I cannot empty the trash. It is stuck on "Preparing to empty the Trash..." for ages (again, we're talking hours).
The .inProgress file in question is 290GB, and is the only file in the trash.

Is there a way to force empty the trash in this situation?

I'm running MacOS Sierra (10.12.6) on a mid-2015 MBP. The hard disk I'm backing up is 1TB, of which 746GB are in use.
I've been through this one and as I recall patience is a virtue, the length of time is linked to the size of the file. I wouldn't recommend forcing it.