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Emyr
USB ports push out a maximum of 500mA, which is enough to power a single laptop HDD (they typically draw ~400mA when running, there's a bit of a spike upon startup but that should be ok).
Your machine may provide 500mA on each USB port, or it may have a lower limit (say 1A total over the 4 USB ports) no way to guess until you try unless you can find the specs.
If you've got a second machine then using that to power two of the drives (those cables from eBay have a separate USB plug for extra power) might work. Any mains USB adapters could be put to use too (like the ones from a Garmin or an iPhone).
Otherwise a powered USB hub (that's rated for 500mA on each port) would be necessary.