Hi Luisr04,
welcome to the Apple (user-to-user) Community.
Re: iPhone Internal Storage Photo Help
Wondering if you also have a computer.
Seems you would use a computer to transfer the files to the 2Tb drive:
Eg: One method:
First
Transfer photos and videos from your iPhone or iPad to your Mac or PC
Then
Simply move a Mac Photos Library to a properly formatted external disk
Move your Photos library to save space on your Mac - Apple Support
Stored this way, seems the Photos can still be viewed with the Photos app.
The iOS photos are accessible on computer, and backed up to the 2Tb Drive.
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Apple support have some other articles online that may be helpful:
You can link the external drive to the Phone to view files:
How to: Connect external storage devices to iPhone – Apple Support
Some apps may allow transfer of files to the external drive:
Eg: article says: "Use a supported app like Files or Pages"
Transfer files between your iPhone and computer with an external storage device – Apple Support
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More support if needed:
Photos Support Website
Photos - Official Apple Support
All the best :-)