MyBook visible - hibernate/wake - invisible - power off & on - visible!

I’m using 2 MyBooks at present - different in age by a year or so.

The older of the two (2009:  320GB Essential Edition) always needs to be unplugged at the mains after hibernation before it will show up in my computer.  Not a real problem, just a pain.

The younger (2010:  1.5TB Essential) drive has no such problems - I wake the PC and it’s there.

Any ideas how to fix this annoyance?

Many thanks ahead