Quickly mount a NTFS disk read/write on all recent OS X (including Yosemite, El Capitan):

1. Open Application -> Utilities -> Terminal
2. Type mount and look for the line with your disk. It will show something like: /dev/disk3s1 on /Volumes/MyDisk (ntfs, local, noowners, nobrowse)
3. Type the following in the Terminal, replacing /dev/diskXsX with your disk as shown in the mount command:
   
   Shell

   sudo mkdir /Volumes/Mount sudo mount -o rw,auto,nobrowse -t ntfs /dev/diskXsX /Volumes/Mount/ open /Volumes/Mount/

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   sudo mkdir /Volumes/Mount   sudo mount -o rw,auto,nobrowse -t ntfs /dev/diskXsX /Volumes/Mount/ open /Volumes/Mount/

To make this change permanent run the following (correct for El Capitan):

1. Run the following command, changing /dev/diskXsX to your disk:
   
   Shell

   export DEVICE=/dev/diskXsX echo UUID=`diskutil info $DEVICE | grep UUID | awk '{print $3}'` none ntfs rw,auto,nobrowse echo LABEL=NTFS none ntfs rw,auto,nobrowse

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   export DEVICE=/dev/diskXsX   echo UUID=`diskutil info $DEVICE | grep UUID | awk '{print $3}'` none ntfs rw,auto,nobrowse echo LABEL=NTFS none ntfs rw,auto,nobrowse

2. Run sudo vifs and paste in the output from the previous 2 lines. To do this press down to go to the bottom of the file, A to start adding text, paste in the 2 lines then press escapeand :wq to write the file. (vifs is the only safe way to edit the fstab in OS X).
3. Run the following to mount the disk
   
   Shell

   sudo umount /Volumes/$DEVICE sudo diskutil mountDisk $DEVICE sudo open `mount | grep $DEVICE | awk '{print $3}'`

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   sudo umount /Volumes/$DEVICE sudo diskutil mountDisk $DEVICE sudo open `mount | grep $DEVICE | awk '{print $3}'`

Note: The device will no longer automatically open a window when you attach it. To access it open a Finder window and select the Go -> Go to Folder to /Volumes