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Does anyone know what determines whether a USB 2.5" external drive will work from just the supplied USB connector? Is it based on the drive's power requirements, the case's bridge-board power requirements, or the supplying computer's USB power requirements?
The reason I ask this is that I built up a drive with enclosure using a spare 80GB SATA 2.5" drive from a Macbook Pro, and the USB-only drive enclosure from OWC. Fortunately I purchased the optional power adapter, as I cannot get the drive+enclosure to mount on either of my two machines. The one is a Sawtooth G4 with a third-party USB 2 card, and the other is a Mac Mini going through a Ministack v2.
I plug the drive in by itself, and the power light comes on, but it will not mount. Only after I add the power adapter will the drive mount.
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> Is it based on the drive's power requirements, the
> case's bridge-board power requirements, or the
> supplying computer's USB power requirements?
Yes.
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USB 1.1 and 2.0 provide +5VDC @ 500mA (2.5W) per port.
If you use an unpowered USB hub/splitter, then the total available power through all the hub ports is limited to the 5VDC@500mA supplied from the upstream port.
If the USB hub is powered, each port will supply +5VDC@500mA .
If the drive + case power requirements exceed 2.5W, a USB port will not provide sufficient power; you will need to provide external power for the case and drive.
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spending extra for firewire would have been worth it and you could have skipped the power adapter
you could also use a dual head USB cable which pulls power from 2 ports
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In the same G4 MDD, if I hook a 2.5" USB HD to the built-in ports, it mounts. If I hook it to a USB2.0 card, it doesn't mount. But this case came with a secondary usb cable for power and when both are hooked to the USB Card, it mounts.
Maybe the USB cards don't provide quite enough power. Haven't tested with a USB2 hub yet as it prevents my G4 from sleeping.
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It's the case and drive power requirements that are the key. OK. Thanks! I chose USB-only as I was trying to save some bucks, the power adapter was only $5. This drive won't be used that much, mostly to shuttle occasional files between my G4, mini, and MacbookPro.
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FireWire is always a sure thing for Bus Power... but the 80GB SATA 2.5" should power via USB no problem on the desktop, the MacBook Pro (one of the ports may be higher power than the other due to the iSight sharing.
Make sure the switch on the enclosure is set to the Bus Power side vs. the AC Power side.. I can't say 100% with the USB2 case off the top of my head, but several of the enclosures will have the drive activity light power on when there is signal but no actual power.
