A USB cable carries 5V, and if this is the sole power source for the drive, it may not be enough.

Check that the drive is receiving enough power.

Make sure your drive doesn’t have or use a power-saving or sleep function.

Try attaching the external drive to another computer to see if it works on that one to rule out damage to the drive itself.

Some imported drives come completely empty, and while macOS should detect and offer to format them, it doesn’t always work.

Make sure the drive has been formatted.

Reboot your Mac to see if it was an open program or app stopping the drive from being detected.

Try a different wall outlet if there is one nearby.

Change the hard drive power cable if you have a spare.

Check the condition of the cable and swap it if you have a spare.

Check that the external hard drive has power if it needs it.

Check that the cable from the hard drive to your Mac is connected properly.

After all, no one wants to waste half a day because they didn’t realize something just wasn’t plugged in properly. Get into the habit of doing this with any computer issues, as it can save time, money, and hassle in the long run.

Troubleshooting an External Drive with a Mac First, we will check the basics, and from there, we make sure that macOS is set up to display external drives, can mount the drive, and that the drive has no errors on either the disk itself or its file structure. If your external drive still doesn’t appear, we have a couple of simple troubleshooting tips to fix an external hard drive not showing up on Mac.

Lacie disk utility