Welcome to the community, casho56!
Can you please give us more information about what you intend to do with the 2.5 TB external HDD? If you wish to have 2 off-site backup copies, you can simply plug your new 3 TB external and copy all the data you wish to transfer. Be patient, though, since 2.5 TB of data could take some time to transfer.
You don't need to uninstall anything, simply use the "Safely Remove" when you disconnect the 2.5 TB external drive.
Keep me posted if you have more questions!
SuperSoph_WD