I agree but i assumed that the 750 did not draws that much more power
(or is more senstive) than the 250 G it replaces. Is there any
software that monitors voltages besides what comes with the Intel
board (which does not alert me to a low votltage state)?
David B is correct. That power supply could have always been
defective - but marginal enough that computer was working. Not
likely that 10 watts from a disk drive would cause significant
problems. However disk drives shut down not because computer shuts
down. Disk drives shut down because the 5 or 12 volts has dropped;
lower voltage indicates to drive that computer is probably turning
off.
There is no way to fix a computer reliably without numbers from a
3.5 digit multimeter. Even numbers from the Intel board are not
reliable until first calibrated by using a meter (just another reason
to get the meter). In your case, the system must be accessing all
peripheral simultaneously (multitasking) when voltages are taken for
any one of orange, red, purple, and yellow wires (from power supply to
motherboard). Those numbers must exceed 3.23, 4.87, or 11.7. And
those numbers may be posted here for further information.
Pictures of where to take measurements:
http://techrepublic.com.com/5102-10586-5566528.html
http://www.helpwithpcs.com/courses/power-supply-basics-inc-pinouts.htm
Lil' Dave also makes comments about y-cables, etc. When the drive
is not seen, then what are 5 and 12 volt on disk drive end of that
cable? (Another reason why information is collected without changing
or breaking any connections.)
Your replies will only be as good as information provided. That
means numbers. Even a supply that claims to be 500 watts could really
only be 350 watts - and the manufacturer would not be lying. There is
no replacement for actual numbers. What it should be is not
necessarily what it is. Get the meter.
First establish integrity of one critical system. It's not just a
power supply. Integrity of the entire power supply 'system' means
taking numbers.