Often times my External Hard disk stops being detected on my MacBook Air. The first few times were scary but now I generally know that I will figure a way out.

Why

It usually happens when the Hard Disk does not get ejected properly. I have read that exFAT on mac is vulnerable to such a scenario and the file system kind of breaks.

How to Fix It

There are two options. The first one is simple: Keep the hard disk connected and let Mac fix it by itself. I have noticed that if you give it approx 10-20 minutes, mac would by itself fix the File System. And optionally you can also try to run First-Aid from Disk Utility

The other option is to use the following steps:

1. Use diskutil list to find the right drive id.
2. You want the id under the IDENTIFIER column, it should look like disk1s1
3. Run sudo fsck_exfat -d <id from above>. eg sudo fsck_exfat -d disk1s3
4. -d is debug so you’ll see all your files output as they’re processed.
5. Answer YES if it gives you the prompt Main boot region needs to be updated. Yes/No?
6. Open Disk Utility and you should be able to repair here successfully.

Source of the above steps