An unreadable SD card could be caused by hardware or software errors. You should identify the culprit and repair it to make the SD card readable again. And it is necessary to recover data from the unreadable SD card given that there is a risk of data loss. Available backups or professional data recovery software could help.
First of all, narrow down the possible causes of the unreadable SD card on Mac or Windows. Check the connection issue, you can change to another compatible and working USB port and USB cable, straightforwardly insert the SD card into the slot without a hub, switch to another SD card reader, etc.
If the SD card still remains unreadable, you should restart your computer, and update OS, disk driver, and firmware to the latest version. Can your computer read the SD card now? If it can't, then the SD card could be the culprit of the issue. It could be a corrupted SD card. You can try the methods below:
- Check the file format of the SD card, it should be compatible with the computer's OS.
- Unhide files on SD card.
- Repair SD card with First Aid of Mac's Disk Utility.
- Repair SD card with CHKDSK command on Windows.
- Reformat the SD card to make it readable again. This will delete all contents, you should recover deleted/lost files from SD card with data recovery software before you reformat it. Try iBoysoft Data Recovery to recover SD card files ASAP!