I think this is because Windows itself is still finalising the writes to the hard drive. I get the same message if I try to disconnect straight after copying some files. If I wait until the drive has stopped spinning then it disconnects without any problem.

I haven't been able to find any information on why this is, so if someone could enlighten us I'd appreciate it.

I think this is because Windows itself is still finalising the writes to the hard drive. I get the same message if I try to disconnect straight after copying some files. If I wait until the drive has stopped spinning then it disconnects without any problem.