I have a couple of external tape drives connected to a server (Ultra320 SCSI). I do not use them frequently, but they are on all the time collecting dust (because of the fans). If I turned them off (without disconnecting them from the bus), without rebooting the server, would it work correctly? Would I then be able to turn them on (when needed) without rebooting? The HBA is LSI Logic PCI-X, but I cannot find the model number (not without opening the server anyway).
SCSI (at least the old-school type for Ultra320) isn't really a plug and play bus, so I don't think it has a direct way for devices to tell the host that devices are leaving or entering the bus. SCSI works by the system scanning for devices when the bus comes up.
But you can rescan the bus at anytime. Try this for Linux, and for Windows, using the "Scan for new hardware changes" in the Device Manager does the same.
You should make sure no SCSI devices are active on the bus before turning the power on/off. If you need to power on/off devices often, put them on their own cable.
Rescan after powering off or on a device.
Bad things could happen if you accidentally power on a device with the same ID as the host adapter.
Avoid hot-plugging SCSI devices unless designed for it, such as hard drives with the hotswap connector.