If Disk Utility sees it only *sometimes*, I lean hardware before macOS. That’s where I slightly part ways with @mikeappsreviewer and @nachtdromer. People love blaming the file system first, but intermittent detection usually screams enclosure, cable, power, or the drive itself being flaky.
A couple things I’d check that haven’t really been covered:
- Open **Console** and watch logs live when you plug it in. Search for `diskarbitrationd`, `I/O error`, `USBMSC`, or `Bridge`. macOS will often tell on the device even when Finder stays useless.
- In Terminal, run:
`log stream --predicate 'eventMessage contains 'disk'' --info`
then reconnect the drive. If you see repeated I/O errors, stop trying random mounts.
- Boot into **Safe Mode** once and test there. Rare, but 3rd party NTFS/paragon/tuxera junk can interfere with mounting.
- If it’s a desktop external with its own power brick, test the **power supply** too. Those fail more often than pepole think.
- If Disk Utility shows the **container** but not the volume correctly, that can point to partition map damage, not just simple corruption.
My order would be:
1. Check logs
2. Test on another machine
3. If readable at all, copy data or scan with **Disk Drill** before repair attempts
4. Only then try mount/repair stuff
Also, if the drive makes your Mac hang when connected, stop. That’s not a cute software problem.
For extra reading, this is a decent thread on **external hard drive not mounting on Mac fixes and data recovery steps**: see real-world fixes for a WD Passport not mounting on Mac