Being it similar to hard drive not showing up on Windows computer, the reasons why your hard drive is not showing up on Mac are caused by the same issues. Here is a list of why a hard drive is not showing up on Mac: Hard drive connection error Settings in Desktop, Finder USB port issue Hard drive corrupted macOS and firmware is outdated ... For a step-by-step guide, you may follow flash drive not showing up on Mac to troubleshoot hard drive not showing up on Mac issue and make the drive detectable again.