It's going to be one of few things:
1) You're not running the 2.02.04 drivers (there is a link above relating to USB Manager).
2) You're running very old 2.04.x firmware. The V8 software not responding will do this as bootblock/firmware prior to 2.5 is not supported by V8. Please see these knowledgebase articles for general information about on-going software and firmware updates:
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Software updates
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Firmware updates
Install the software updates first as the V8 software incorporates the new bootblock and firmware updates for your device.
If you have very old 2.04.x firmware please see the bottom section of the firmware updates article. If you are on this very old release it is imperative that you first upgrade to 2.07.72 prior to upgrading to the later bootblock and firmware versions. This is outlined in the document.
3) Cable or device fault. You indicate you've tried a different cable so that can be ruled out. You could get the device sent in for inspection/repair but that will cost you shipping and possible an inspection fee and repair costs so last resort.
4) You have some very odd system issue that is just going to be too difficult to diagnose.
My money is on
1) or
2) personally.
Cheers,