Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Protect your External Hard Drive
If you use your external hard drive to store some confidential data, you may want to protect it from prying eyes of other people. One of the ways to protect your files is to encrypt them. In Mac OS X, this can be done with Disk Utility.
1. In Disk Utility, click File, New, Blank Disk Image.
2. In the Save As field, enter the name for your disk image.
3. From the Where drop-down menu, select your external hard drive.
4. From the Volume Size drop-down menu, select one of the predefined sizes, or click Custom and enter the needed size in the Custom volume size field.
5. From the Encryption drop-down menu, select either 128-bit AES Encryption or 256-bit AES Encryption. 128-bit AES Encryption is less secure but faster, therefore it’s up to you to decide which encryption method to choose.
6. From the Image Format drop-down menu, select sparse disk image.
7. When ready, click Create and enter the password that will be used to access your disk image.
8. Move the files from your external hard drive to this image.
If for some reason you don’t want to use Disk Utility, there’s a plenty of easy-to-use data encrypting apps that will hide and encrypt any file or folder you like.