Do you own a Mac? Have you purchased an external hard drive and are not able to put files on it? Fear not, this week I’ll teach you how to solve this little mystery.

The reason for this problem is the drive is formatted using the Windows proprietary NTFS file system. Macs cannot write to this natively. The simplest solution, if it is a brand new drive, is to reformat the drive, making sure to select the MS-DOS (FAT) option (this will make it so both your Mac Windows computers can read it).

To reformat: Open the Applications folder, and within Utilities, choose Disk Utility. Start it up and you’ll see all the drives listed on the left side. Click on your new drive. You’ll want to click on the “Erase” option along the top after you’ve picked the drive. Now you can pick which format should be used when the disk is rebuilt from the list. Click “Erase”.