If Time Machine on your Mac can’t find your backup disk
Try any of these suggestions if Time Machine can’t find your backup disk.
If your backup disk is connected to your Mac, make sure the disk is firmly connected, plugged into a power source and turned on.
If your backup disk is connected to a network, check the Network section in the Finder to make sure the backup disk is shown. Make sure the disk is turned on and that Time Machine support is enabled on the server. If the network is experiencing issues, the server may need to be restarted.
If your backup disk is connected to another computer, make sure the computer isn’t in Sleep mode. If it is, disable the sleep function for future backups.
Open Time Machine settings and reselect your backup disk. If you erased your backup disk and began having trouble, reselecting it may fix the problem.
If you’re using a Time Capsule as your backup disk, use AirPort Utility to make sure it’s set up properly and connected to your network.
Click the Finder icon in the Dock to open a Finder window, then check the Locations section in the sidebar. Your backup disk should be listed. If it’s not, it may be turned off, defective or currently unavailable on the network.
If you back up to a Time Capsule or other network disk that has a password, the password may have changed. Log in to your Mac as an administrator, choose Apple menu > System Settings > General, then click Time Machine . Reselect the backup disk, then enter the new password.