DS stands for 'direct scsi', (though it might be possible to use it on some IDE controllers too)
it is used to bypass some limits, i think. (like 4Gb, or maybe 8Gb, I keep forgetting.. thankfully there's Thomas to remind us.. :-) )
edit: from the manual
If your device does not support TD64 then you'll have to use the @{"direct scsi" link "DirectSCSI"}
Normally PFS3 uses the standard device interface to access the underlying
device. If this device is the "scsi.device" you can choose to bypass the
standard device interface and have PFS3 send direct scsi commands to the
device. This is what the direct scsi version does. It can be used for any
scsi device. The advantage of using direct scsi calls and the reason PFS3
offers it is that it enables PFS3 to circumvent limitations of the standard
device interface. There is NO speed difference between the normal and the
Direct SCSI versions.
When to use the direct scsi version.
Using direct scsi makes it possible to use disks larger than 4GB on which
@{"TD64" Link "TD64"} is not supported. Some devices already show problems if the
device is larger than 2GB. If you experience such problems, try the direct
scsi version. Warning: direct scsi may not function correctly on all systems
and controllers due to bugs in the scsi.device.
ONLY use the Direct SCSI version when your controller doesn't support TD64
and your harddisk is larger than 4GB.
Why not to use the direct scsi version.
The functions of the standard interface are best adapted to access the
device and can deal with problems of the used scsi device. PFS3 is not
and cannot be aware of the specific problems of all scsi devices which
could cause problems in some cases. Therefore don't use the direct scsi
version if you don't have to.
Note: Direct scsi can also be used for the internal IDE controller of the
A4000 and A1200 (this IDE controller is accessed by an emulated scsi
controller which supports direct scsi calls). DS works great on those
internal IDE controllers.
