1 Sure you can
2 Yes, but you will need additional SATA controller to connect more drives and something like this or this to place them inside G5 case.
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The maximum internal storage capacity of a G5 should be 18TB with proper mounting.
5 3.5" drives (2TB) and 8 2.5" drives (1TB).
Of course all that drives would require additional SATA (RAID) controllers.
Not exactly sure what power supply the G5 has (700watt?), but leaving it running 24/7 as a NAS will cost you significantly more than a 2 or 4 drive dedicated NAS unit. It'll be like running a small microwave oven continuously.
Not exactly sure what power supply the G5 has (700watt?), but leaving it running 24/7 as a NAS will cost you significantly more than a 2 or 4 drive dedicated NAS unit. It'll be like running a small microwave oven continuously.