@nikolajhendel:"mAh rating sholdn't really make a difference regarding working with the ext HD. Higher mAh shold only mean that they pack mpre charge = last longer.
Regarding number of batteries, you only need four. They are connected to deliver a 5 v charge.
6 or 8 batteries would give too high a charge"
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Some yes and some no. There is a threshold to kick start a HD, I think it is called "peak surge" and there is juice to do work, "continuous load". When the threshold is reached, a continuous spinning HD may continual to spin, but once it stops, it will not be able to get up. So, a lower mA battery extender may stopped much earlier than the juice is actually finished.

You probably think ALL USB AC adapter are born equal, right? I do, AC adapter means unlimited juice. Well, I was dead wrong. USB adapter that rated low, I got a couple, one rated 200-300 mA, another 350 mA, are NOT able to drive my HD. The one rated 1A easily drive the HD.

I have a battery inverter, that has 2 outlets. one produces household current, 115V AC and another USB outlet of 5V 350 mA DC. The 115 V outlet can drive a big 3.5 in 750G HD, but the dumb 350 mA USB outlet is NOT able to spin the tiny palmsize HD.

4X battery gives about 5.5V, you are right, they do not have to do anything about the V. For 6X or 8X that would give much higher than 5V one may have to do some 'step down' conversion. But I have eyes on the ampere. I guess an electrical engineer may be better to judge whether that is a good proposal.

@qole:"My old laptop spontaneously rebooted when driving my USB HD! I think the HD drew too much power... "
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I used to think so. In your case, if it made your laptop reboot, the power consumption logic is proabably valid. As I play with battery more, I start to believe, power consumption is only part of the game, threshold rating is also a factor.

bun