Does the source code (dump.asm) try to include a file from the 'M' drive at all?
EDIT: Ah, RTFM! You do not specify the file extension...
If you do (.asm in your example) it doesn't do what you think it does...
The individual letters 'a', 's' and 'm' are taken as three parameters:
'a' - the disk drive containing the source file.
's' - the disk drive to receive the assembled object/hex file.
'm' - the disk drive to receive the assembler listing file.
So, the assembler is trying to store the listing file on drive M:...
Omit the '.asm' file extension - or use it to specify the parameters to ASM to correctly assemble your file.
See
http://www.cpm.z80.de/randyfiles/DRI/ASM.pdf pages 1 and 2.
By the way, welcome to VCFED.
Dave