I'll give a summary here, but search the archive for Tony Duell's postings on 9114 drives, the mechanism of the 9121is similar to that of the 9114A with the exception that the 9121 is SINGLE sided.
Unlike modern floppy drives, it takes DOUBLE density diskettes (rather than quad density). QD diskettes have
a hole across from the READ-ONLY tab. The 9121 has a "floppy present" sensor over that spot, so if you insert a QD floppy it will not be detected by the drive. According to Tony this is a good thing as QD floppies have different properties from DD floppies, so they are not really interchangable. So you should look for DD floppies on eBay rather than try to use QD diskettes from your PC.
Having said that, you can generally get away with QD diskettes if you block the hole with a piece of tape. Since the sensor is mechanical, transparent tape works.
To format the diskette try
INITIALIZE "label",":D700"
for the left hand drive and
INITIALIZE "label",":D701"
for the right hand drive ("label" can be any string up to 6 characters long)
To copy a file from one diskette to the other, try
COPY "name:D700" TO ":D701"
note that if the source diskette has volume name label1 you can also use:
COPY "name.label1" TO ":D701"
To copy ALL files use:
COPY ":D700" TO ":D701"
**vp