help on rgdal package

I started to use the rgdal package in order to work with DEM files. Since
these files are based on non-rectangular quads, there are lots of
NA-values in the corresponding datasets. Does anybody know how to remove
the NA-values from an object of class GDALDataset in order to get a
rectangular dataset?

Also, the function saveDataset doesn't seem to work for these kind of data
(or at least for me). I get the following error:

x <- new("GDALDataset", "xxx.dem")
getDriverLongName(getDriver(x))

[1] "USGS Optional ASCII DEM"

saveDataset(x,"xxx.data")

Error in .local(.Object, ...) :
        GDAL Error 6: GDALDriver::Create() ... no create method implemented for this format.

Or should the dataset be raster data? Then I get:

rd<-getRasterData(x)
saveDataset(rd,"test.data")

Error in .assertClass(dataset, "GDALReadOnlyDataset") :
        Object is not a member of class GDALReadOnlyDataset

Does anybody see what am I doing wrong? Thanks.

Hana Sevcikova