[ESS] "An indirect buffer cannot visit a file."

[Christopher W Ryan]

Sometimes when editing an .Rnw file, working with R, Sweave, and
emacs, I want to save my current buffer under some other file name
than the file it came from, and sometimes I will get this error
message:

"An indirect buffer cannot visit a file."

I have not yet found a patter as to the conditions under which this
error message pops up.

I'm having a hard time understanding indirect buffers, but I've never
intentionally used or created one. I don't anticipate I will ever have
a need to. I don't know how a particular buffer becomes an indirect
buffer. Is there an indication somewhere, perhaps in the status bar,
that a buffer is an indirect one? And is there a way to turn an
indirect buffer back into a "regular" buffer?

Thanks.

--Chris Ryan