Problem solved! With the help of the Italian WD support, and some ingenuity on my side, the drive is working again.
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I let the drive off for a day, and then waited for its LEDs to flash slowly (standby).
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I connected it to my Mac (Mavericks) via USB.
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Run Disk Utility, and formatted the volume (not the drive).
It worked. When reconnecting the My Book Studio II via Firewire 800, I could copy the previous content from a backup. Then I let Time Machine back it up again. At the moment, it seems to work fine again.
What cause this issue? It happened while I was doing nothing with the drive. Maybe it was still the old incompatibility between WD Drive Utility and Mavericks, caused by the WDDriveUtilityHelper app I found running from the /private/var/ directory?
Paolo
