just found an answer to the differences. there is a partial 7902/9895 manual hp_floppydisk_7209xx.pdf on dyndns.org that has the history -- Introduction This service manual contains service information for the 7902A, 9895K and the 7902C flexible disc drives and the controller boards used on the HP 250, 300, 1000, and 3000 series 30 and 33 systems. The following brief history describes the changes that have taken place on the disc drives and controller boards and the reasons for the changes. The first production model 7902A flexible disc drives were installed in the HP 250, 300 and 3000 systems. The 7902A drives (P/N 07902-68811) designed for use in the HP 250 and the 7902A drives (P/N 07902-60038/07902-60023) for use in the 300/3000 were non- interchangeable. For this reason a common drive (P/N 07902-67914) was developed as a replacement. Refer to service notes HP 250-01 and 7902A-01 for instructions if one of the old drives is discovered in the field. In the past, the drive mechanisms for the 7902A had been manufactured by Shugart and utilized a tri-compliant head design. As of September, 1980, this drive was replaced with one using a bi-compliant head design used on the 7902C. The drive used on the 9895K is built by CDC and utilizes a tri-compliant head design. These drives are not interchangeable between systems because of differences in drive boards used on the various systems. The drive boards used on the HP 300/3000 systems are designed and built by HP and utilize control signals which are non-industry standard. The drive boards used on the HP 250 systems are designed and built by both Shugart and CDC and utilize industry standard control signals. The controller board (P/N 45000-66510) used on the old HP 250 systems, and the control- ler board (P/N 07902-60024) used on the old HP 300/1000/3000 systems, were also non- interchangeable and they both utilized in MC2 micro CPU. A new controller board (P/N 07902-6652(1) was designed as a replacement for the 45000-66510 board. If one should fail in the field, refer to service note 07902A-5A for instructions. The controller board used on the old HP 300/1000/3000 systems (P/N 07902-60024) is interchangeable with the new controller board (P/N 07902-66501) designed for use with the 7902C. The new HP 3000 systems no longer use the 7902 disc drives. The 9895A flexible disc drives will be used in place of the 7902.