Many large CoCo programs were provided on multiple floppy disks but were designed to work on systems that had only one floppy drive. These programs will ask you to insert a specific disk into drive 0 as it is needed.
Using multi-disk programs with the CoCo SDC requires a folder (directory) to be created on the SD card. Inside this folder you place all of the program's individual disk image files. Each image file must be given a name that ends with a unique numeric character (1 through 9). The figure below shows how the three-disk game "Sinistaar" could be setup on the SD card.
A folder setup in this way represents a disk set. To use the disk set you provide the path name of the folder as the mount-point in a DRIVE command. A disk set can only be mounted in drive 0.
Mounting a disk set will initially mount the image file which has the lowest numbered name (SINSTAR1.DSK in this example). To switch to a different disk in the set you press the button on the CoCo SDC board. Each press of the button will rotate through the numbered image files, mounting each one in turn. Upon releasing the button the LED will blink a number of times to indicate which image has been selected.