I keep getting an error message in LR CC when I back up that the drive where LR is stored is full and urgently needs 40MB space freed up. In fact, it is a 1TB external hard drive with more than 400MB free. LR eventually backs up but I'm bothered by this error message. (Windows 7)
1) Close all applications, shutdown your system, and turn the AC power OFF to ALL connected devices including the 1 TB external HDD.
2) Disconnect phones, cameras, and other connected devices with storage that aren't needed, except the 1 TB external drive.
2) Wait for at least 30 seconds and the turn the AC power back ON for all devices.
3) Reboot and try the LR Backup again.
If you still have the issue please provide the model number for 1 TB external HDD.