arcgis desktop - What causes RuntimeError: ERROR 010240 saving after CellStatistics?

I am having trouble saving outputs from CellStatistics but only when I run it on a list of rasters. This was working on python 26 but not 27.

I know this example is nonsense, but:

env.workspace =    "G:\\Faculty\\Mann\\Historic_BCM\\Aggregated1080\\" 

relevant_tifnames_year_i= ['cwd1975jan.tif', 'cwd1975feb.tif', 'cwd1975mar.tif', 'cwd1975apr.tif', 'cwd1975may.tif', 'cwd1975jun.tif', 'cwd1975jul.tif', 'cwd1975aug.tif', 'cwd1975sep.tif', 'cwd1975oct.tif', 'cwd1975nov.tif', 'cwd1975dec.tif']

mean = CellStatistics(relevant_tifnames_year_i, "MEAN", "DATA")
mean.save(str(env.workspace)+"outa.tif")

sd = CellStatistics(relevant_tifnames_year_i, "STD", "DATA")
sd.save(str(env.workspace)+"outb.tif")

Atest= CellStatistics([mean,sd], "MEAN", "DATA")
Atest.save(str(env.workspace)+"outc.tif")


Traceback (most recent call last):
  File "", line 1, in 
    Atest.save(str(env.workspace)+"out3.tif")
RuntimeError: ERROR 010240: Could not save raster dataset to G:\Faculty\Mann\Historic_BCM\Aggregated1080\Scratch\meanc_ras3 with output format GRID.

Notice that I can save CellStats run on a list of files from my working directory, but not on a list made up of my intermediate inputs "sd" and "mean". I know that I can run it if I pass the file paths, but in my actual code I need to run it off of intermediate inputs. Am I missing something here?

Note: Overwrite = True has been set. There are no files with the same name in the output folder etc.

Here is the actual code (storing rasters to a list and then running CellStats)

for year in base_year:

    # paths for files
    relevant_tifnames_year = [v for v in relevant_tifnames if int(re.sub('[^0-9]','',v) or 0) >= year and int(re.sub('[^0-9]','',v) or 0) <= year+years_to_end ]
    #list to store annual means
    decadal_holder_Amn = [] 

    # for each period of interest
    for year_i in range(year, (year+years_to_end+1)):
       # get relevant paths
       relevant_tifnames_year_i = filter(lambda x: str(year_i) in x,relevant_tifnames_year)
       #calc annual mean

       mean = CellStatistics(relevant_tifnames_year_i, "MEAN", "DATA")
       #append mean mean to list of annual means
       decadal_holder_Amn.append(mean)
    #calc mean of annual means for the decade
    Amean=CellStatistics(decadal_holder_Amn, "MEAN", "DATA")
    Amean.save(str(env.workspace)+"outd.tif")

Answer

Amazingly all my troubles have to do with the lack of gdb use. As soon as I assigned the scratchWorkspace to a .gdb my write errors started going away!

env.scratchWorkspace ="G:\\Faculty\\Mann\\Historic_BCM\\Aggregated1080\\Scratch.gdb"