I stored the important data on my external hard drive, one day, when attempting to access it and I get a message: "The disk is unreadable, do you want to initialize the disk?"
If you are using the latest macOS High Sierra, the above error message will be: "The disk you inserted was not readable by this computer with three options: Initialize, Ignore, or Eject?"
What causes "The disk is unreadable error" on USB flash drive, SD card, external hard drive?
"The disk is unreadable" message usually means that the directory of the drive has become corrupted. Corruption can happen for a number of reasons:
Catalog file corruption
Internal file system errors
Disconnecting or power off the drive without unmounting it
Forcing a restart on the computer
Experiencing a software lockup while writing to the drive
Turning the computer off without doing a proper shut down
Solution 1: Fix "Disk is unreadable" error with macOS inbuilt Disk Utility
macOS built-in disk utility has the ability to fix unreadable/unmountable disk error, please open Applications -> Utilities -> Disk Utility -> Select the unreable disk -> Run First Aid. if First Aid fails to repair unreadable disk, please go to solution 2 to recover data from unreadable disk.
Solution 2: Recover data from unreadable drive before fixing "Disk is unreadable" error
Due to the above causes, Mac Operating System fails to read the hard drive and the data stored in it becomes inaccessible. To access the hard drive again, maybe you want to run First Aid in Disk Utility. If you are experiencing the disk is unreadable error and you have a backup of your data elsewhere, use the Disk Utility to erase the drive and then restore your data from backup. If you do not have a backup, you need to use a professional Mac data recovery software to restore data.
What's worse, when encountering "The disk is unreadable by this computer" or "The disk you inserted was not readable by this computer", you clicked "Initialzie" button to initialize the unreadable disk without knowing this operation would completely delete all your precious data from the hard drive, then in that case it is hard to recover lost data. So you need a professional Mac data recovery software.
Tutorial to recover data from unreadable hard drive, USB flash drive, SD card in Mac
Step 1: Download, install and launch M3 Mac Data Recovery on your Mac computer.
Step 2: Select "In a specific hard drive" option and then select unreadable drive from drop-down list.
Step 3: M3 Mac Data Recovery is scanning the data from unreadable drive.
Step 4: Select the found files you expect to recover and then click Recover to save them.
Tips: If "In a specific hard drive" option fails, please try "In a specific volume" option.
Solution 3: Fix "The disk is unreadable" error by reformatting
After data recovery is successfully done, you can reformat this unreadable hard drive, external hard drive, SD card, USB flash drive, memory card, CF card, etc. to fix "The disk is unreadable by this computer" or "The disk you inserted was not readable by this computer" error in macOS 10.13/10.12 Sierra and Mac OS X 10.11 (El Capitan), 10.10 (Yosemite), 10.9 (Mavericks), 10.8 (Mountain Lion), 10.7 (Lion) with the following solution:
Step 1: Launch Disk Utility from Utilities.
Step 2: Click the icon for your hard drive in the sidebar on the left.
Step 3: Click the Erase tab along the top of the window and enter the related information.
Step 4: Click the Erase button.
Step 4: Click the Erase button to fix "The disk you inserted was not readable by this computer" error. After format completes, the unreadable drive will be working again.
Solution 4: Look help from a local data recovery professional
If disk is still unreadable by any Mac data recovery software, please send the unreadable disk to a local data recovery professional for help if you lost important data.