Spinning ball, can't save, can't relaunch PTLE, reboot then drive unreadable!

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As per the title, has happened twice now.

PTLE 8.0.4, running an external FireWire drive from the Digi002R.

Simple session, around 12 x 30sec clips, 2 soft synths, duplicate a simple kick drum MIDI loop several times. Hit play...

...spinning ball for a few minutes, finally clears but can't save (-50 error), close PTLE, but session nor back-up sessions won't load (-9126 error), reboot computer to discover hard drive now won't mount, OS and Disk Utility says drive unreadable and needs to be initialised.

This is the second time this has happened recently since updating to 8.0.4. I don't normally run an external drive from the Digi002R, but need to while my rig is downsized for an impending move across the country.

Any ideas?

Kinda scary, and thank goodness for (semi-) recent drive back-ups.