Glen & Christain, many thanks for the information, and the pointer to the 9895A service manual at http://www.hpmuseum.net/exhibit.php?hwdoc=262The manual also has this to say: p.37:Before reading or writing a sector of data, the controller must position the read /write head of the disc drive over the desired sector. To do this, the controller sends signals to the disc drive to step the head to the desired track. The controller then begins to read the disc. Track and sector information is stored as a preamble to each sector, and the disc is formatted in a manner to allow the controller to detect the beginning of a preamble. On finding a preamble, the Z80A first checks that the track number is correct. If not, it will continue to step the head until the correct track is found. p.43 has figure 2-3 Track Format. This shows each sector consists of (ignoring gaps):ID Field made up of:- Sync up Bytes- Address Mark (AM) [in other manuals refered to as IDAM]- ID Information (bytes indicating the logical track, head and sector address)- CRCData field made up of:- Sync up bytes- Address mark [in other manuals known as Data Address Mark or DAM]- Data- CRC p.45 Address MarksID AM : Data pattern = !70Defective Track AM : Data pattern = !F0 At the command set level, a clear D bit indicates an ID AM in the ID field, and a set D bit indicates a defective track AM in the ID field. p.86 Start of Appendix A, 9895A Disc Memory Command Set p.89 - has description of the D bit. p.95 Shows the Command Table. p.105 'Request Logical Address' Purpose: The 9895A returns bytes indicating the current target address. This command is used to determine the address of the offending sector after a data error has occurred. p.106 'Request Physical Address'.Purpose: The 9895A returns bytes indicating the physical cylinder on which the head actuator is positioned. This is useful for calculating the number of invisible tracks between the outer-most track and the current track. This is done by subtracting the physical cylinder address from the target cylinder address. P1.31 Describes the Initialize command, and P.133 the Format command. Together these sections describe use of the D but and invisible tracks. To conclude, information such as the IDAM content won't be readable by a PC with a normal FDC, so this approach looks limited. I am in touch with Ansgar reading his wonderful hpdir project, which I am using with my Series 80 disks on an 82901M drive, so will point him here for a bit more light reading ;-) Regards,John