Unison sorts changes in order to first add new files and then to delete removed files.
When I use it to synchronize 2 USB drives, I can have problems with that if there's no room enough in the drives to store new staff without deleting first removed stuff.
So how can I tell unison to perform first deletion and to do anything remains?
Apologies for disagreeing, but I don't think Unison behaves like you are describing. According to the Unison manual on this page, Unison uses rsync with the
--inplace
option as the default copy utility so that updated data is written directly to its destination. You could try specifying these defaults in your profile to see if that helps.