How to fix external hard drive 'Not detected' or 'can't transfer data (Read-only error)' on Mac

Base on the different defective reasons, please try below solutions to solve the problem:

1. Check whether the USB cable has been damaged.
2. Change another USB port and check whether the USB port has bad contact.
3. Check whether the external hard drive can be recognized by Windows.

If your external hard drive is NTFS as the file systems, it cannot be accepted by MacOS. You could reformat the drive to compatible format with built-in Disk Utility on Mac by following steps below: 

• Launch Finder →[ Disk Utility] →[Applications]→ [Utilities] 
• In the list of available drives on the left, please choose the problematic external hard drive and click [Erase]

• [Format] :Choose a proper file system. [Scheme]: Choose GUID partition map to reformatting external hard drive.

※Both APFS and Mac OS extended file system are only compatible with Mac. If you want your external hard drive to be both Mac and Windows compatible, please choose ExFAT instead, or MS-DOS (FAT32) but the size limitation of one single file cannot be over 4GB.

※Please back up data in your external hard drive before reformatting. 

• Click [Erase] to begin formatting your external hard drive. After the reformatting process to complete, then click [Done] when finished.