Unfortunately, same problem here.
I have one iMac (27', late 2009, 3.06 GHz Intel Core 2 Duo, 4GB), one Macbook Air (11', late 2010, 1.4 GHz Intel Core 2 Duo, 4GB), and two 1TB external drives formatted as Windows NT.
After upgrading both Macs' OS to Lion successfully, the two external drives cannot be mounted onto the iMac, while working fine on the Macbook Air. In the Disk Utility of the iMac, both two external drives can be still observed.
I have tried to reset the PRAM and SMC, but no luck so far.
The only difference when upgrading OS is that when the iMac was being upgraded, one of the external drive was connected.
I also tried Marco.Cantu.Gea's method, and it works. I am going to call Apple to see if they are aware of this issue.
Good luck everyone.