I have an external FireWire disk that booted with 10.7 using legacy FileVault, which is what 10.7 calls the FileVault installed by 10.6. MJ would not finish the install ... it popped up a message that it was already installed.
Bottom line is that it appears you can't install mJ on a drive with legacy FileVault. I have not tested 10.7 FileVault.
MagicJack won't work with legacy FileVault
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