Error building buildSignatureMatrixUsingSeurat

[spatts14]

I can build my matrix with scRNAseq reference with 12 out of the 28, but I feel getting this error while doing buildSignatureMatrixUsingSeurat at the same point. The connection doesn't actually seem broken as it keeps "breaking" at the same point. I know I can read and write into the file because it has successfully done it 12 file before creating the signature for 12 other cell types.
Any ideas?

Error in file(file, ifelse(append, "a", "w")) :
cannot open the connection
Calls: buildSignatureMatrixMAST ... write.csv -> eval.parent -> eval -> eval -> -> file
In addition: There were 35 warnings (use warnings() to see them)
Execution halted