Strange floppy disk boot problem on Dimension R series

​ I recently upgraded a Dimension D series with an R series motherboard. I needed to flash the bios so dug out my range of Optiplex boot disks and updated a copy of these with the R series bios flash progam. However, no matter what I did, the R series would not boot from the same disks as all my GX1 Optiplexes boot from - it would hang before the DOS prompt appeared. Swapping floppy disk drives and cables did not cure the problem, nor did using PCDOS boot disks and Win 98/ME/XP created ones. FInally, in desperation I created a DOS 6.22 boot disk and that worked flawlessly. Anyone experienced this behaviour before or can explain it? ​