Migrate information.

Hi,

I'm trying to migrate but the "export" process it's very slow (about 12 hours to export 60 GB and there's about 400 gb).
Looks like there's a read issue on the old server.

It's possible only to migrate domains/users information ?
What happens if I try to import a backup but the domains and users already exists on the "new" server and have information ?

Thanks.