External drives · Mounting and drive problems
What does "The disk you attached was not readable by this computer" mean?
Short answer: macOS can see the drive but can't read its contents: it's unformatted, in a format a Mac doesn't read, or damaged. Initialize erases it — fine for a new drive, but copy anything you need off it first (on the computer that can read it) if it holds data.
macOS found the drive but can't make sense of what's on it. It's either brand new and unformatted, formatted in a way macOS can't read (such as a Linux file system), or its file system is damaged. The three buttons mean:
- Initialize — opens Disk Utility so you can erase and format the drive. This wipes everything on it.
- Ignore — leaves the drive connected but unused.
- Eject — disconnects it.
Before you initialize, ask whether the drive holds anything you need. If it came from another computer, read it there first and copy the files off. If it was working on your Mac until recently, try First Aid in Disk Utility and another cable or port — a damaged connection can look like an unreadable disk.
For a new, empty drive, Initialize is the right choice: erase it as APFS for use with your Mac. See which format to use.