It is being used by other process, or Quickbooks errors 6073,618:

This may happened due to what dear MS called "Opportunistic Locking." That is Server 2003 that the WHS is sitting on, place a lock on a file upon request from the software (like Quickbooks) on a client computer.

To avoid this, you have to tell the Server 2003 to DENY such requests.

You do this by adding / modifying registry value.

Read more here: http://support.microsoft.com/kb/296264/en-us

Here is is how its done:

Open registry, and Navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\
LanmanServer\Parameters

In there, either add (if absent) or modify (if present) the below value:

EnableOplocks (REG_DWORD) should be 0 (the Default: 1, enabled)

The EnableOplocks entry configures Windows-based servers to allow or to deny opportunistic locks on local files. These servers include workstations that share files.