The error sounds like an earlier logged issue we have, SPR GFLY6X9RME, which reported that the Full Text Search Engine did not accept \\UNC filenames. This is an enhancement to review support for that.
Check to see whether you are using a UNC path structure, if so, try the workaround noted in the following technote,
Title: Full-text index creation fails with "Error full text indexing document NT00000000 <path/database.ft> (rc=309) Invalid drive" when using UNC path
Doc #: 1420497
URL: http://www.ibm.com/support/docview.wss?uid=swg21420497