The ".grd is not a valid grid file" error message indicates that you are selecting a GRD file format that is not supported in Surfer. The GRD file extension is shared by a number of different grid file formats. Alternatively, the file may not actually be a grid file. If you believe it is a supported grid file format and it is still generating the error, please zip and attach the file to a support request so we can take a look and determine what the issue is.
Another potential cause of this error is that you are attempting to access a file on a network that uses aggressive file locking. Older versions of the Novell network operating system had the problem and Novell issued a patch. Consult your network administrator or vendor for additional information. To work around the problem, copy the file to a local hard drive prior to opening it.
Updated February, 2021