Make sure the user (usually hts) owns the recording folder.
chown hts:hts /my/recording/folder -R
chmod 755 /my/recrding/folder
Avoid saving to the sdcard and mount a small usb powered drive or network storage if you can.
if that still fails do a test recording and post the logs from the debug window here (click the bar on the webui at the bottom)
HTH