SQL Errors
First of all, you should not be receiving SQL Error 18054 (or any SQL errors) from TFS.
You should have your TFS administrator connect to the SQL server that hosts the master DB for your TFS server and run the following query:
select * from master.dbo.sysmessages where error > 50000
If this is a TFS2010 server, your TFS administrator may be able to use TFSConfig PrepSql to re-install the error messages.
If this is a TFS2008 server, your TFS administrator will need to open Add/Remove programs and run a repair on TFS.
Your actual problem
This sounds obvious at first, two
local paths cannot point to the same
place in the repository for the same
workspace. However, the one that
catches a lot of folks un-aware is
that you cannot have two repository
paths mapped to one local path on the
same computer.
In TFS, you cannot have two folders with overlapping mappings. Since D:\aaa\bbb\ccc\ffffd is a sub-folder of D:\aaa, then you cannot add it.
One thing you can do though, is cloak folders so that they aren't part of the workspace mappings. In your case, you might want to map D:\aaa and add a cloak for all the other subfolders in that directory, except for D:\aaa\bbb.
