LaCie 500GB d2 Quadra 500 GB USB 2.0 Hard Drive will not mount. Little green light on the power cord blinks. I unplug it from the back of the external hard drive and it turns solid green. Tried a different power supply from similar LaCie, green light goes out when plugged into the hard drive, and I know it's a good power supply that I'm trying. Disk does not spin or click.

If drive has any warrenty, then send it to company or the dealer. If no warrenty then the power section of your hdd casing have problem. open the casing and remove hard disks and back up your data first. and then do other necessary procedures. best of luck

Hello,
if the unit has the original hard drives inside (i never one with 10 000 rpm drives though), the problem might be caused by a power issue.
Please verify the power supply by plugging it into the wall then listening to it. It should be silent. If there is a high pitch noise or hissing, it is defective and needs to be replaced.
If replacing the defective power supply did not solve your issue or if the power supply did not have any issue at all, the internal components will need to be inspected.
If you replaced the internal drives with 10 000 RPM drives, it might be why the drives do not get enough power.
Regards,

Hello,
you will need to go in Disk Management and format the drive first then it will be available in My Computer.
Regards,

Try pluging the device to another Pc to see if the device works

Same here. I pulled the drive from the case and swapped it with another and the drive works fine. Must be the control board inside the case. Where can we find the replacement board?

if the harddisk is making clicking sound,, then there may be an failure of motor inside it,,, ,, nothing but giving your HD for replacement by manufacturer

Modern hard drive motors can easily become seized after an impact. The only DIY solution I am aware of is the use of a heat gun over the back of the drive motor. The build up of heat within the motor is sometimes enough to free it.

If the drive was running the problem may be due to a phenomenon known as "sticktion" where the drive head is actually stuck to the disc surface. If this is true then you need a data recovery company.

This is most probably a faulty power cable, a common fault on La Cie apparently. Another cable got my disk working fine! 

Contact Lacie support for a new power supply. I had the same problem, contacted Lacie support, they sent me a new power supply with power cord and all is well now.

Maybe the usb chip in the hard drive enclosure that converts ide into usb failed. You could replace the enclosure (they sell enclosures at best buy) but it could be quite a hassle.