For an individual file, if you are copying from the current directory on A: to the current directory on C: you would specify the file name and extension:
copy a:filename.ext c:
or if you need to specify the directory, it is like this:
copy a:\folder\filename.ext c:\folder\folder\
Then if you want to copy all the files in the current directory on A: to the current directory on C: it is like this:
copy a:*.* c:
If your command prompt is in the destination directory on C: already, to copy the files found in the current directory on A: you could simply type:
copy a:*.*
If your command prompt is in the destination directory on C: already, if you want to copy all the files and the subdirectories from the current directory on A: then I think the command is:
copy a:*.* /s