best external hard drive for macbook time machine

To use the HDD for both your data and Time Machine, create an additional APFS volume, using one for data, and the other for Time Machine. This is a change from earlier versions of Mac OS that supported only HFS+ for Time Machine backups, and explains why Mac OS lets you store both data and Time Machine backups on the old HDD: it's not APFS formatted.

So the problem is that, for a new external HDD, Time Machine requires an APFS formatted volume, and APFS volumes cannot be used for Time Machine and user data. The two volumes then share the available space. If you want to store files other than the Time Machine backup on the same physical device, use Disk Utility to create an additional APFS volume on the disk. Note: The entire APFS volume is reserved for Time Machine backups. If the disk is a Mac OS Extended format disk that contains an existing Time Machine backup, you aren’t asked to erase and reformat the disk. If you select a new backup disk that’s not already formatted as an APFS disk, you get the option to erase and reformat it.

APFS or APFS Encrypted disks are the preferred format for a Time Machine backup disk.

Best external hard drive for macbook time machine