Problem solved. The megasas floppy wasn't being read at install because of the way I created it. I finally solved it by downloading in binary format and using rawrite to create the floppy in DOS.
I have done all that but the new install fails. I guess I assumed that the floppy was not created properly in OS5.0.7 but maybe that's not the problem after...
I have a new Dell PowerEdge 2900 with perc5/i and no os. I need to install SCO Openserver 6 and I need create a megasas BTLD floppy in order to install the os.
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