imgmount is for mounting images *not* devices. Why don't you mount the device in linux and then mount the volume in Dosbox?

Thanks for the quick reply!

What's the difference in this case between an image and a device file? They are both (in my case) 4GB files, just on different underlying storage devices. If I open a 4GB file or /dev/sdd in my hex editor, they are identical in size and content. I can mount a 4GB file in DOSBox then copy it over the top of /dev/sdd to use the SD card in a real machine, so I am just hoping to skip the copy step and have DOSBox access the SD card directly, just like any other Linux program does when you tell it to operate on the file /dev/sdd.

I'm not sure what you mean by mounting the device and then mounting the volume. I want to run FDISK on the device within DOSBox, which means it can't be mounted under Linux, unless I'm missing something? I don't think you normally mount .img files in Linux before passing them to DOSBox, so this wouldn't be any different as I'm attempting to do the same thing.

I had a look at the DOSBox source code and I discovered it is a bug in the way the image file's attributes are checked. DOSBox thinks the device file is a directory when it is not. Using the correct check function solves the problem and I am able to mount my SD card directly in DOSBox.

1diff -ru dosbox.orig/src/dos/dos_programs.cpp dosbox/src/dos/dos_programs.cpp
2--- dosbox.orig/src/dos/dos_programs.cpp        2015-05-02 19:31:20.867019736 +1000
3+++ dosbox/src/dos/dos_programs.cpp     2015-12-18 22:08:22.422102325 +1000
4@@ -1216,7 +1216,7 @@                           
5                                                }
6                                        }
7                                }
8-                               if ((test.st_mode & S_IFDIR)) {
9+                               if (S_ISDIR(test.st_mode)) {
10                                        WriteOut(MSG_Get("PROGRAM_IMGMOUNT_MOUNT"));
11                                        return;
12                                }
13@@ -1377,6 +1377,10 @@  
14                        
15                        } else {
16                                FILE *newDisk = fopen(temp_line.c_str(), "rb+");
17+                               if (!newDisk) {
18+                                       WriteOut(MSG_Get("PROGRAM_IMGMOUNT_FILE_NOT_FOUND"));
19+                                       return;
20+                               }
21                                fseek(newDisk,0L, SEEK_END);
22                                imagesize = (ftell(newDisk) / 1024);