That's normal. You can't mount drives after using the boot command, and any drives that aren't images are unavailable.

Thanks for your prompt answer.
Got it, but how can I create a "shared" volume to transfer things back and fourth then? The C drive image can't be mounted on my host OS.

And apparently you can't have two HD images mounted at the same time... perhaps I'm wrong though.

Do any copying before running the boot command.

OK. I've commented out the boot command from the autoexec section of the config file and managed to mount both C and D drives at startup (without booting into DOS of course)

1imgmount c ~/Documents/DOSBOX/C_DRIVE.sparseimage  -size 512,63,32,520 -t hdd
2MOUNT D ~/Documents/DOSBOX/D_DRIVE
3D:

I'd like to run the following commands at will:

1MOUNT -u c
2imgmount 2 ~/Documents/DOSBOX/C_DRIVE.sparseimage -size 512,63,32,520 -t hdd -fs none
3boot ~/Documents/DOSBOX/C_DRIVE.sparseimage -l c

NB: I need to unmount the c drive and remount the image as drive '2' otherwise the whole thing'll crash.

My idea was that I could write a boot.bat file with those commands that I could invoke by typing boot in the console, but apparently I can't use .bat files on dosbox?

You can run bat files but only from mounted drives.
Or you can use the autoexec section in the config, or pass a user config from a shortcut

Turns out converting an .rtf file to .txt and then to .bat does funny things. I used a windows machine to create the batch file and now it works. Yay.
Also, I realized that "boot" was already used by dosbox so I named the .bat file dos.bat, which once invoked starts ms-dos.
That way, I can do whatever I have to do to manage my files from D to C and once I'm done I can start MS-DOS in a breeze.