Agreed, it is probably best to just define the fftemp folder yourself.
Maybe some background will help:
ff is a package for keeping objects that are too big to fit in memory. It does this through ‘memory-mapping’: part of the data is kept in memory, and part is written to a temporary file on the disk. To the user, it looks as if the object is entirely in memory, and your operating system swaps out what is on the disk with what is in memory as needed.
That means that every time you open an ff object, it needs to create a temp file. It does this in the fftempdir. As long as the ff object is open, the file is locked and you cannot change it with any other program.
If you restart R but keep your workspace, things get a bit tricky. Maybe the temp file was cleaned up (deleted), but you still have an R object that points to it, so that could be one reason you get this message. Another reason for trouble could of course be if there’s no space left in the fftempdir, but you’ve already ruled that out.
I find it helps to once in a while just close down all instances of R, and delete your workspace (called “.Rdata”). And specify your fftempdir as Chan mentioned.