Terminal: On wake, mount external disk automatically

"Try mounting the disk itself, rather then a single volume (even if it's the only volume on the disk).

For your .wakeup script, replace

diskutil mount 8851F3A7-60A7-39A7-9DA8-41BE1499DE6C

with:

diskutil mountDisk disk1s3

... replacing 'disk1s3' with the proper device identifier for your own external disk, of course. I don't think I need to tell someone who is capable of specifying a volume by its UUID where to find that information. For everyone else, here's how:

diskutil list"