Imagine a SCSI device like a CD-ROM changer, loaded with 5 CD-ROMs. Now imagine that a SCSI host controller is requesting data from CD 3. The cd-changer would have to unload the current CD-ROM, select CD 3 from its rack, load the CD, start spinning, read the data, and transmit it back to the host controller. This would obviously take quite some time.During this time, all SCSI devices in the system would be waiting for the CD changer to complete its job; no other SCSI transfers would be possible during this time.However, when a SCSI device supports disconnect/reconnect, it could disconnect itself from the SCSI bus, do whatever business it needs to do, and reconnect to the SCSI bus when it's ready with the requested data.This allows other SCSI devices (e.g. fast Hard Drives) to continue transferring data, while simultaneously allowing the slow device to process its data in its own way.