I have run init 0 to bring it to OK prompt.

I know that probe-scsi-all can hang the system.

I want to know what is the difference between probe-scsi-all and probe-scsi.

I was told that probe-scsi-all external and internal devices and probe-scsi shows only internal devices. what dose this external and internal devices mean?

And is there any way to prevent this hang?

probe-scsi identifies SCSI devices attached to only the built-in SCSI bus. The system is not hanging, it is probing for scsi targets.

Actually that is not correct, they system may and probably will hang if you do a probe-scsi or probe-scsi-all after an init 0 (or other OS shutdown command) or stop+A followed by a probe-scsi or probe-scsi, in fact it will warn you not to do so. The difference between the two commands is however correct.

The correct way to do probe-scsi or probe-scsi-all is:

go to the ok prompt ( depends what you're doing how you get here, but you already know how to get there so no worries there )

{ok} setenv auto-boot? false
{ok} reset-all

(system reboot to ok prompt )

{ok} probe-scsi-all
....
some output
....
Then to get it back to normal:
{ok} setenv auto-boot? true
{ok} boot

Thanks a lot guys.
I really appreciate it.