When you try to open an external hard drive on your Mac, you may get the message that "The disk you inserted was not readable by this computer."
That means your external hard drive has become unreadable. That may happen to anyone. When you click "Initialize", you will lose all your data in that external hard drive. If you click "Ignore" nothing will be changed, and you still cannot read that external hard drive. You can choose "Eject", and unplug it, then plug and mount it, and try to open it again. However, the message will appear in front of you again.
External hard drive unreadable can be caused by virus infection, category file corruption, internal file system errors, improper ways of handing the external drive or Mac, etc.
You can use Disk Utility to fix this issue. But sometimes, it doesn't work. You can rebuild the external hard drive to make it readable. However, that will cause data loss. In this situation, professional Mac data recovery software is needed. iBoysoft Mac Data Recovery is highly recommended to recover data from unreadable external hard drive on Mac.
Recover data from unreadable Mac external hard drive with iBoysoft Mac Data Recovery
iBoysoft Mac Data Recovery is professional and safe software that helps to recover lost/deleted documents, photos, videos, emails, music, etc. from hard drive, external hard drive, USB drive, flash drive, SD card, CF card, memory card, micro card, digital camera & other storage device in macOS 10.14 (Mojave), macOS 10.13 (High Sierra), macOS 10.12 (Sierra) and Mac OS X 10.11/10.10/10.9/10.8/10.7. It works well to recover data from unreadable Mac external hard drive.
You can recover data from unreadable Mac external hard drive with 5 simple steps.
Steps for recovering data from unreadable Mac external hard drive with iBoysoft Mac Data Recovery
Step 1: Download and install iBoysoft Mac Data Recovery on your Mac.
Step 2: Connect unreadable external hard drive to Mac.
Step 3: Launch iBoysoft Mac Data Recovery and choose the unreadable Mac external hard drive.
Step 4: Click "Next" to start scanning data in that external hard drive.
Step 5: Preview the scanning results, choose those you needed and then click "Recover" button to recover those data.
Besides recovering your data from unreadable Mac external hard drive, you can recover data from formatted drive, inaccessible hard drive, invalid volume header hard drive, unmountable hard drive, recover data from formatted external hard drive, recover data from corrupted external hard drive, etc. with iBoysoft Mac Data Recovery.
After recovering your data from that unreadable Mac external hard drive, you can launch Disk Utility and run "Erase" to format that unreadable Mac external hard drive and make it readable.