CC3200: Trouble writing /sys/mcuimgA.bin to flash

I'm adding custom OTA capability from a TFTP server.  During the update process, I'm opening the file in flash but getting an error returned (-23) from sl_extlib_FlcWriteFile when I write.

I've created /sys/mcuimg2.bin and /sys/mcuimg3.bin in the file system via uniflash.

Here's the relevant code:

		lRetVal = sl_extlib_FlcOpenFile ("\sys\mcuimgA.bin", 200000, 0, pFileHandle, FS_MODE_OPEN_WRITE);
		if(lRetVal!=0)
			{
			Report("Error opening file on flash: %i\r\n",lRetVal);
			}

		lRetVal = sl_extlib_FlcWriteFile (pFileHandle, 0, pbuffer, buf_size);
		if(lRetVal!=0)
			{
			Report("Error writing to file in flash: %i\r\n",lRetVal);
			}

		lRetVal = sl_extlib_FlcCloseFile (pFileHandle, 0, 0, 0);
		if(lRetVal!=0)
			{
			Report("Error closing file: %i\r\n",lRetVal);
			}

What is sl_extlib_FlcWriteFile return -23? 

Any ideas what could be the problem?

Thanks,

Alex