\setbeamertemplate{footline}{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{author in head/foot}\insertshortauthor~~(\insertshortinstitute)
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
\usebeamerfont{title in head/foot}\insertshorttitle
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
\insertframenumber{} \hspace*{2ex} % hier hat's sich geändert
\end{beamercolorbox}}%
\vskip0pt%
}
\documentclass[t]{beamer}
\usepackage[absolute,overlay]{textpos}
\setbeamertemplate{navigation symbols}{}
\def\insertpresentationendframe{\inserttotalframenumber}
\makeatletter
\g@addto@macro{\appendix}{\immediate\write\@auxout{\string\@writefile{nav}{\noexpand\headcommand{\noexpand\def\noexpand\insertpresentationendframe{\the\c@framenumber}}}}}
\makeatother
\setbeamertemplate{footline}{%
\begin{picture}(54,12.5)(0,0)
\put(0.9,0.52){%
\begin{minipage}[b][12.5mm][c]{112.5mm}
\raggedleft
\insertframenumber/\insertpresentationendframe
\end{minipage}
}
\end{picture}
}
\begin{document}
\begin{frame}
slide in the main part
\only<2>{blub}
\end{frame}
\appendix
\section*{Backup}
\begin{frame}
\frametitle{backup}
not counting in the total frame number
\end{frame}
\end{document}
\documentclass[t]{beamer}
\setbeamertemplate{footline}{
\hfill%
\usebeamercolor[fg]{page number in head/foot}%
\usebeamerfont{page number in head/foot}%
\setbeamertemplate{page number in head/foot}[appendixframenumber]%
\usebeamertemplate*{page number in head/foot}\kern1em\vskip2pt%
}
\begin{document}
\begin{frame}
slide in the main part
\only<2>{blub}
\end{frame}
\appendix
\section*{Backup}
\begin{frame}
\frametitle{backup}
not counting in the total frame number
\end{frame}
\end{document}