The SCSI bus requires termination power to provide clean, quality SCSI
signals. When a SCSI device has its own termination power, the SCSI bus
signals are corrupted when the device is off because this leaves the SCSI
bus unterminated. An non-terminated SCSI bus can cause startup problems
because of noise on the bus.
Sometimes, a SCSI device supplying its own termination power can remain off
at startup because it has a relatively clean SCSI bus signal. But there is
no guarantee that the bus signals will stay clean and uncorrupted, as it is
improperly terminated in this situation.
SCSI devices that use Macintosh termination power can be off and still be
properly terminated.