There is currently a kludge in out RemoteFilename handling that performs the translation of c$ to c:, but only if part of the URL is non-posix.
e.g. //x.x.x.x\c$/path/x would be translate to c:/path/x and succeed to write to c:\path\x via a windows dafilesrv.
However //x.x.x.x/c$/path/x or or
x.x.x.x/c$/path/x would not be translated, nor would dafilesrv perform any translation of /c$/ , so consequently, files would be written to c:\c$\path\x
The translation should be handled by dafilesrv not client code which doesn't really know how or what will use the URL. Separate issue opened: HPCC-18555.
For this fix, always translate a URL with a leading /c$ to a drive letter.