Prohibited sign - Can't boot after trying to open Raspian SD Card So I was following this tutorial https://www.youtube.com/watch?v=Eo4HphBdR10 and opened the folder of the SD Card. The Screen went black and I rebooted.

Now I get a Prohibited Sign on boot up.

In the Recovery mode (cmd + options + r) I see that my Macintosh HD is unmounted and repairing sais that something is wrong with the Disk.

I can't reinstall because the installer isn't finding any Disks.

What should I do ?
Oh, drat! Sorry to say you most likely lost it all.

This is the exact reason NO one should just follow any random video on the Web. To explain what I am saying, there is a high probability you used diskutil improperly with the wrong disk identifier (your Startup Disk rather than the intended target) and as root, which most likely overwrote Mac OS X with Raspbian without mercy. What is odd about that, however, is that Recovery does work.

For future reference, let me give you a better way to write Apple supported images. Open Disk Utility and click your flash drive. Go to the Restore tab and drag the image in the source box. In Yosemite and later, go to Edit > Restore and browse for the image you intend to restore instead. Restore. But, our problem is this won't work for Raspbian, as it's a Gnu/Linux flavor, not the standard Apple dmg. So we will need Terminal. 

But first, a few notes: Whatever you decide to do things using Terminal, be very careful (especially as root, and that goes for all UNIX (and *nix-like systems!) The sad part is that the guy in the video was close, but didn't do a god job of clarifying the inherent danger in blindly writing an image with all powers. ALWAYS back up first before running a dangerous command.

Now, as for the method. If you're new to all this, I would avoid playing in diskutil. Instead, use mount with no arguments to find your disk identifier. You'll note your hard disk also has an identifier, which for example could be /dev/disk0. If you know the name of your flash drive, that's even better as we can filter that right out and ensure we have the right one. So, if your flash drive or card's name is example, we can run mount | grep example to filter what we want. So, now, if it's disk3, you'll see /dev/disk3s1 and example all nicely listed and filtered out, being automounted for us under Volumes.

Once you are absolutely certain what the identifier (or ending number) is for your flash drive, go ahead and type if= and drag the image, so it pastes the path into the Terminal from the Finder. Now apply our previous finding and add of= (output file). Again, if mount or Disk Utility gave you /dev/disk3s1 for example as the flash drive, /dev/disk3 is what we need (we want the root of the drive, not the ending s# which stand for partitions in this scenario). But, this could be different on your system (check first!). In our example here, the completed command might look something like this: dd if=/Users/me/Downloads/2015-11-21-raspbian-jessie-lite.img of=/dev/disk3. Or, let's give an alternate example where it was disk5 and we went to the right directory (like, say, Downloads), we could then autocomplete with tab, add the if, and do this: dd if=2015-11-21-raspbian-jessie-lite.img of=/dev/disk5

But in either case, back up first. Be careful. Do great.

Now, the pressing and more challenging question is how to get your data back. You already discovered your startup disk is gone. First, try restoring from a Time Machine backup (assuming you wisely had this set up). If not, try writing a Linux Mint image (I would say Fedora, but dracut sometimes hangs on live USB in my experience since R19) using a Mac with the example above, or another UNIX (or like) system (though there will be some differences; you will probably see /dev/sd# or mmcblk# for some cards on modern Gnu/Linux flavors, for instance). Or, download win32imagewriter for Windows XP SP2 or later and use that with Administrator rights (again, watch for the drive identifier first; in this case, the letter!) Assuming you have Mint booted, run parted (if it is not available, connect to a network, sudo apt-get update &amp;&amp; sudo apt-get install parted -y. What we are hoping for is that the table is still somewhat intact. Do sudo -s to elevate then run parted -s -l. You should see /dev/sda or whatever your hard drive is.

If you have a valid GPT table and you don't get any warnings about that, continue. If not, skip this paragraph. In the case of sda, I'd do parted /dev/sda. Now, from what you observed earlier, look for the end of any preceding partitions (most likely where EFI sat). Now, assuming a default El Capitan setup, or anything past Lion, this should be 209.7 MB. The start should therefore round close to that point (if the GPT isn't too awful corrupted, parted should warn you if your query does not match and try to readjust). For the end point, where does Recovery start (usually volume 3?) For instance, on my 160 GB disk, it is after 42.6 Gb. This is our end point. With an insane amount of luck, try putting a start and end for rescue, such as rescue 210M 42.6GB and see what happens. If this failed, even with an adjustment, you could try fdisk. The problem, though, is that this is generally for undoing changes before a reboot that are still fresh; this is only a long shot.

If you didn't back up anything, you're not entirely out of luck just yet. From a NetBoot, FireWire, USB 3, or Lightning external startup disk (I.e. that has OS X on it), you can still use recovery tools such as Recuva for Mac, Prosoft DataRescue, or DiskWarrior to try to salvage whatever you can. Raspbian most likely overwrote some of those files good, but if you have the extra time and cash, it'll be worth it.

If all else fails, or if the table is corrupt, your best bet would be to forget about Recovery (since its on the same disk) and reinstall entirely. Ideally, you'll need to either own or borrow another Mac for this next step. Download 10.11, close the Install OS X pop up window that appears, and use createinstallmedia from inside the OS X installer application bundle/package per the instructions from Apple (there is a guide for this). Power down your Mac, hold Option, and boot from the disk. You'll then need to go to Disk Utility and create a new Intel or GUID partition table (GPT). Create a new partition for your data and the OS; OS X should do the rest when it installs. Close Disk Utility and install as usual. Restore whatever you had backed up.

In any case, I'm hoping all this is unnecessary and that you had Time Machine, iCloud, or another backup system working for you. Good luck with it all. Careful with root commands in the future. Hope this helps!