Strange. You should be able to copy all files you have read access to a folder on the external disk you have write access to. And that's both in Windows and in Linux (you didn't tell what OS that backup server has).
So maybe you don't have read access to all files. Try running the copy as administrator.
i use simple sharing within my network.
there is a dedicated backup server, providing access to all computers in the network.
all the computers backup to this backup server using a file compare/copy tool.
so far, everything works fine...
to have an off-site backup, i would like to backup these data to an external disk.
unfortunately, this does not work. copy to this external disk starts, but after a few files, it locks, reporting an access violation.
how do i have to configure windows to allow the server to backup files, other computers have saved to the server?