What file system in on the external drive? Many drive manufacturers have drives formatted as NTFS as a convenience to their Windows users, but macOS is only able to read an NTFS volume by default although you could install the manufacturer's proprietary third party software to allow writing to an NTFS volume. However, as @jeffreythefrog mentions, that proprietary software sometimes can cause problems (especially when macOS receives any update patches or OS upgrades).
If you need to share the drive with a Windows computer, then it is better to use Disk Utility on macOS to erase the whole physical drive as GUID partition and exFAT which both macOS & Windows can access without requiring the use of problematic third party proprietary software.
If you will only use this drive with Macs, then it is best to erase the whole physical drive with either APFS or MacOS Extended (Journaled) file systems. Which one depends on whether the drive is a Hard Drive or SSD, and also which versions of macOS need access to the data on this drive.
- For a Hard Drive use MacOS Extended (Journaled) since the APFS file system can cause performance issues & wear on a Hard Drive. For an SSD either file system is Ok, but APFS is better suited for an SSD to allow better performance & it reduces the wear on an SSD.
- If you need to share this drive with an older Mac running macOS 10.12 or earlier, then use MacOS Extended (Journaled) file system.
Here is an Apple article with instructions for erasing the whole physical drive:
Erase and reformat a storage device in Disk Utility on Mac - Apple Support
FYI, no matter what file system you use, make sure the partition type is GUID when erasing a Hard Drive or SSD.
