How do i send something, as in a writer document to an external USB drive (64GB)?
Do you know how to use a computer?
Not really a Libreoffice related question.
Open the file explorer, make sure that the USB drive is plugged in (it will be visible in the overview). Copy the file and paste it in the USB device.
From the LO menu: File - Save a Copy - Browse your USB drive - Save it.
Note: never work on a removable media directly. You may lost your important documents when you do an irregular removing procedure of the USB device.
The “Save a Copy” feature will not redirect the URL of the actually edited file to the USB device. It will create an independent copy of your document. You still edit the original one. Do not use the “Save as” feature for this, because it will redirect the URL to the USB device, and you will edit the copy.