Lenovo IdeaCentre H520g Hardware Maintenance Manual page 68

d. Extended video memory
e. External Cache
f. External Cache RAM
g. Hard disk drive
h. Diskette drive
3. Turn on the power and the computer to re-test the system.
4. Repeat steps 1 through 3 until you find the failing device or adapter cards.
If all devices and adapter cards have been removed, and the problem continues, replace the system board.
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Lenovo H520gHardware Maintenance Manual