> All these four devices have DMA modes. Why only the first one is working
> with DMA ?
> hdb, hdc and hdd are all "DMA disabled" during boot.
Linux is currently automatically using DMA only when DMA mode 2 is
configured at boot (*sword2 or *mword2 in the "hdparm -i" output).
For all other cases (including ATAPI devices), one can enable bus
mastering DMA transfers manually using "hdparm -d1 /dev/hdx".
Gadi