Hi, Sorry for the long post. I have done a lot of things to make this work but am still stuck on this problem. As some of you know, I and some others have been busily working on restoring some Vector Graphic machines. One goal is to replace the Tandon TM100-4M 100tpi drives with more commonly available High Density 96tpi floppy drives. I realize the disks between the two types of drives are incompatible and I plan to retain both types of drives for compatibility with other Vector Graphic machines. However, the 100tpi drives are difficult to obtain and I would like to limit their use to preserve their remaining lifespans. I would like to transition my regular usage disks to the HD 96tpi drives to utilitize commonly available and inexpensive HD floppy drive mechanisms. I believe it is possible to make an HD drive emulate a legacy DSQD drive -- at least I am sure it can be done on the PC. The late Don Maslin was apparently able to use 720K (DSQD) 5.25" floppy drives on his Vector Graphic system (see link below). I would like to do a similar thing but using the HD drives in DSQD emulation mode. http://www.classiccmp.org/pipermail/cctalk/1998-January/093114.html I have a couple of HD floppy disk drives such as the Panasonic JU-475 and TEAC FD-55GFR. I have jumpered and modified both drives so that the spindle motor turns at 300 RPM which I have verified using a fluorescent lamp and tach wheel. I have set the jumpers or directly wired to ground Pin 2 to force Double Density mode. I have covered Pin 2 on the interface with tape so that it does not interfere with the FDC on my Vector Graphic machine and vice versa. According to the schematic of my Vector Graphic integrated FD/HD controller (FDC) neither pin 2 (density select) nor pin 34 (/ready) are used. After many tests and inspections, I have verified both HD drives do work in 720K DSQD emulation mode on my floppy test bench PC. They are configured as 3.5" 80 track drives. The motor spins at 300 RPM and they are both able to format and verify with scandisk under MS-DOS using good quality 5.25" disks. The problem comes in when I attach either HD drive to the Vector Graphic FDC. Whenever it tries to format under CP/M, the VG is able to write out all 154 tracks (I have verified it is writing the tracks by inspecting the disks using my Catweasel) but the VG cannot verify the tracks correctly and fails the format. Everytime it tries to format a disk, it ends with "PERMANENT DISK ERROR" messages. Using different CP/M utilities to write to the HD drives results in similar errors such as VG BACKUP, a disk copy utility, and CPYFSTU, another CP/M disk copy utility. I do not know for certain whether the disks are have errors when written or the drive is having problems reading the track. I know the disks are good as I can format them on the VG using the TM100-4M drive without issue. When I verified the tracks are written using the Catweasel and the same disk and drive I noticed several read errors and bad sectors which implies the tracks were written with errors. Does anyone know why these HD floppy drives are incompatible with the Vector Graphic FDC? Is there something I am missing or some setting which would make them compatible? The HD drives work just fine at DSQD drives on the floppy test bench PC so I am fairly sure the drives themselves are OK. The VG FDC works fine with the TM100-4M drives so I think they work OK. The problem is only when I attach the HD drives to the VG FDC. Would anyone please give me some insight on this problem? Thank you in advance for any help. Andrew Lynch