I have the pst files on a separate partition that I had to format low level because of bad clusters. I created the partition again and reloaded the pst and other files but then a different drive letter had been assigned. The original drive letter was given to a cd -rom drive. When I tried to change the drive letters back to their original position in computer management the cd-rom drive had a different letter from the one in windows explorer.
The original letter was not available.
So now I am stuck. Any body knows how to get out of this pitfall?

I have solved the problem. It happened to be a hardware problem. The dvd writer that kept the required driveletter so stubbornly appeared to be defective. I have temporarely replaced this writer with an old cd-rom player allowing me to switch the drive letters by the usual method.