NeXT '040 Cubes

Hi there, I have two cubes, both original 1988 boxes. One is a 25MHz '040 with a hard drive and an optical drive. The other is a 33MHz '040 (Turbo?) with just a hard drive. Both power on fine, do their system test and then say loading from network. The picture looks like a coaxial "T" connector with a cable that has "data" rolling through it. They will not do anything else. I've been going crazy looking on the internet for days, but have not found a way to bypass this. I know you can "boot" into single user mode and then reset passwords, but how? Anybody lend me some suggestions? Keyboard shortcuts, etc. Also, I have an external 4gb hard drive that I have tried to connect to a working NeXT slab that I have, but it always has a SCSI error - no matter the SCSI ID or with or without terminator. Shouldn't I be able to use this with a NeXT and build a disk? Then couldn't I boot from the external if needed and build the internals? Thanks for any help, they sure are cool systems, x-N! ASA. Best, David