"Device not ready" on a network share in Windows 7

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http://serverfault.com – I have two computers C1 and C2 runing Windows 7 and both of them are members in a domain. On C1 I have an USB hard-disk which I shared for the users U1 and U2 giving them Read-Only permissions on the entire drive. However, even if I can see and browse the hard-disk localy (IOW from C1), from the other computer (C2) where I'm logged with U1, trying to access the C1's shared device, the C2's Windows 7 throws an error saying "Device Not Ready". Why? How can I fix this? PS: Tried to un-share and re-share again. No luck. (HowTos)