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I've got a Seagate FreeAgent GoFlex Desk 2TB External USB 3.0/2.0 Hard Drive that I use for backups.

I previously had issues with USB ports not supplying sufficient power to the drive, but that was resolved by plugging into back-plate ports on back of case, rather than case ports. Now it does not connect/mount when plugged into the same port(s) in the back of case, but the drive isn't bad cause it's fully functional on USB 2.0 ports.

Drivers haven't changed and don't show any issues, so I can't really explain this one. Disk Management doesn't pick it up either, and not as unallocated space as one might expect. In all my experience with computers, they don't just suddenly change their ways unless prompted to do so.

How can I troubleshoot this further?

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So it shows up when connected to the rear USB 2.0 ports but not USB 3.0 ones? Have you tried on multiple PCs, or is this behaviour restricted to a single machine? – Karan Jan 16 '13 at 6:16
Confirmed only on one other laptop, in which case it's not surprising power might've been not enough. What's strange is no, it works on both modular case top USB 2.0 ports, as well as the rear ones. For this reason, I couldn't understand why the drive can work on case top USB 2.0 ports, but not the 3.0 case top ports, since if anything they should be allocated more wattage, not less. However, as stated in initial query, the drive has ceased functioning on ANY USB 3.0 ports, both case-top and back panel. – Daniel Jan 16 '13 at 20:49
I have this same excat problem. Non-USB 3.0 drivers work in the 3.0 slot too. Running Linux Mint 14, just for the record. – Mister Dood Apr 4 '13 at 4:15

Check the device manager to see if any unrecognized devices are attached to the computer through USB or if there are any devices that are listed as having issues. This would indicate a problem with the SATA to USB bridge.

The next thing I would try would be using a USB 2.0 cable to connect to the HDD to the USB 3.0 port. This should force it to communicate over the USB 2.0 standard. I also recommend running Seagate SeaTools to check the health of the drive. The majority of its power should be pulled through the power supply of the drive and not through the USB as it is a 3.5” external hard drive.

It does not sound like a power problem, likely an issue with the cable being broken and not allowing communication at USB 3.0 speeds, or the chip in the external HDD enclosure.

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