I hate to over simplify, but it needs to be covered. Start here, then consider more complex steps.
Physical troubleshooting.
The symptoms indicate there is a problem with the drive, how it is connected, or with the computer used for troubleshooting. The next best step is to change physical variables, and find a simpler problem than a broken product.
~ Change the data cable (USB, FireWire, Ethernet, etc.) A failure in the cable can cause this issue.
~ Change the external power supply or cable. If a different LaCie product is available, borrow the power cabling from that unit to see if the problem is in that component. On drives that are powered via USB or Firewire, try using the included power sharing cable and connect it to a second port.
~ Change the computer you are using. Computer problems can manifest themselves in many ways. Even if the computer appears to be working properly, this is a worthwhile test.
If you have the same results on another computer with both drives I would have to assume both drives were connected to your computer during the upgrade and the upgrade corrupted the data. ~TE
My LaCie d2 and WD external hard drives are invisible since I upgraded to mountain lion OS. I tried to restore/ verify them in Disk Utilities; both apper in grey and error message 'disk not mounted' appear. Both connecetd to mac via USB. Any suggestions on how to rectify this?