Deleted Data Mark Sectors:

There are several pieces of information the 1771 disk controller (in the 810) attaches to each sector. They are internal to the operating system of the 810, and are invisible to the main computer. The "data mark" is one such data item. The normal value of this mark is $FB (1111 1011), which is assigned by the resident program of the 810 via the 1771 controller. What is done is that the data mark is changed to the so-called "deleted data mark" value, which is $F8 (1111 1000). The software manufacturer, again through modified hardware, will assign the deleted data mark to various sectors on a disk, much the same as a bad sector.