The new Seagate drive might be pre-formatted as NTFS for Windows, which is a read-only file system on Mac, therefore, you can't copy/move files from Mac to the Seagate drive. You can directly format it in Disk Utility to make the drive writable.
When you get a new drive formatted NTFS and you want to use it for Mac, then, you can format the external drive on Mac and choose a macOS-supported file system, such as APFS, HFS+, exFAT, and FAT32, then you can copy/move files from Mac to external hard drive without limitations.
In case you get an in-use NTFS drive in use and you don't want to reformat it for macOS, then, you can try NTFS for Mac driver such as iBoysoft NTFS for Mac to read and write NTFS on Mac.
