EXTERNAL SCSI DEVICE DISAPPEARING

HI, I have a client W2000 PRO Ita SP2 with an adaptec 2920a and two external device connected, an Iomega 1 Gb jazz (scsi number 4) and a Fujitsu Dynamo 640 SE, magneto optical drive, able to read and write optical floppy of 640 an below (scsi number3). First they work perfectly, but sometimes they vanished from the computer, and if somebody click on the icon of the drive it says or “semaphore period expired”, or sometimes “device not connected”. Sometimes a reboot is able to revive them, sometimes only to turn off the machine, and sometimes neither that.
Any ideas?
Thank You for the attention
Paolo Fascia