DVD does not work in a Panasonic hard drive DVD recorder

I have a panasonic DMR-EX87 hard drive DVD recorder. When I put a pre recorded DVD in the 'hatch' it says on its small display 'NO READ' and on the TV screen 'Cannot read. Please check the disk'. This happened after an aborted attempt to record from hard drive to DVD. I cannot get the DVD to behave normally. I have reformatted hard drive, restored factory settings and tried with a number of different DVDs.