DDI it's a disk-image file created by the MS-DOS program Diskdupe.
 It seems to be a [.DSK] file with a 4608 byte header.
 
[These information found at http://www.faq.msxnet.org/suffix.html]
 
This program by Sean Young converts a DDI disk image to a DSK one.

source code taken from:
http://www.funet.fi/pub/msx/othersys/ddi2dsk.c

Morphos conversion by Fabrizio "Lanch" Bartoloni (lanch@tiscali.it)

 PERMSSN    UID  GID    PACKED    SIZE  RATIO     CRC       STAMP          NAME
---------- ----------- ------- ------- ------ ---------- ------------ -------------
[generic]                 3867    9648  40.1% -lh5- adcb Jun  7 22:13 ddi2dsk/ddi2dsk
[generic]                  882    1937  45.5% -lh5- 5aca Jun  7 20:06 ddi2dsk/ddi2dsk.c
[generic]                  370     621  59.6% -lh5- 773d Jun  7 22:50 ddi2dsk/ddi2dsk.readme
---------- ----------- ------- ------- ------ ---------- ------------ -------------
 Total         3 files    5119   12206  41.9%            Jun  9 19:02