Importing new photos into a new Lacie 4TB external hard drive

You didn't mention how you formatted the new drive. To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

You might also want to check the permissions for your new drive, if you're sure it's properly formatted.

Ignore Ownership should be checked.

Docbruce wrote: New external hard drive won't erase. This message pops up.

That's frustrating. Background processes are running. Try unplugging the drive while logging out or restarting. Then get Disk Utility to take control as you plug it in.

With Disk Utility open Control (right) - click on the LaCie and expect it. Then go back to DU and try erasing again and format it APFS if an SSD or OS X Extended (journaled) if an HDD.