When I run Update Manager I get this error:
This is what the error box says:
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://linux.dropbox.com lucid Release: The following signatures were invalid: NODATA 1 NODATA 2
W: Failed to fetch http://linux.dropbox.com/ubuntu/dists/lucid/Release
W: Some index files failed to download, they have been ignored, or old ones used instead.
You need to add the public key for the Dropbox repository, instructions from https://www.dropbox.com/downloading?os=lnx:
sudo apt-get update
You don't need to be too worried about that error. It simply means that some files could not be found (probably the keys that are used to "sign" the repository). If you added the keys, then don't worry about it. If you have not added them however, you may need to get them from http://linux.dropbox.com/ and add them to your system.
In Software Sources GO to authentication and restore defaults. Type the following in terminal: