External back-up drive

The use the drive as a Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS.

Normally - Time Machine Backup likes the Entire Drive for that sole purpose and nothing else.

Use the drive for multi-purposes is akin to putting all the eggs in one basket. If the Drive fails - the Backups and all the dear and precious Photos are gone.

The Drive would need to be formatted as HFS Journaled with the GUID Partition Map. This will wipe ALL the Data off the drive. So make sure to transfer the existing files off the drive before formatting.

Most reliable makers of External Drives will function as long as the female USB ports on this computer match up with the Male USB Cable that comes with the External Drive. That is if the computer has the older USB-A female ports the external drive cable needs to have USB-A Male cable. So goes for the newer USB-C port to USB-C cable.

As for Out of the Box and no reformatting - most makers of the drives tend to format for a Windows Market meaning the drive may be formatted as FAT32 with the Master Boot Record Partition Map or even as exFat with the same Partition Map. Still other may even format as newer Windows NTFS Format and bundled their Software on the drive ( as New _ with Special Software so Apple can read and write to the NTFS file format.

If the drive is ONLY to be used on the Apple Computer - it is suggested to NOT use any of the above files formats and for sure Do NOT use the bundled software as it tends to be buggy.

To use a native file format for Apple Computers - it would be best to Reformat the Entire Drive to the Native Apple File Format.

To achieve this - attach the drive to the computer. Disk Utilities >> View >> View ALL attached Drives. 

2 - Choose the Upper Most Drive by the Name of the Manufacturer - example Seagate, Western Digital etc..

3 - That is the drive to Erase and format as HFS Journaled with the GUID Partition Map. 

4 - Format and give it a name of your choice.

5 - Done.

It is not a good idea to use a drive for backup and as normal storage. A backup drive should be used for backups only.

It is always a good idea to format a new drive. This removes all data on the drive and prepares it for use. If a drive already has data on it then you format the drive all data currently on the drive will be lost.