some motherboards have a jumper to force the network boot. Sounds like this is what's happening, especially if the puter came from a large business.
tinstaafl
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Once you have an operating system installed, if the network book has a lower priority, then it will not attempt because the OS will load first.
However, in your case, if you have no OS, then the computer will go through your boot priority list and try each option until all have failed.
JorgeM
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