Hi grandlido50, my sister purchased a batch from Tesco recently and had the same problem. She returned them. BUT there is more to this story. I have a 6500A (2 in fact) as well that will occasionally refuse to recognise media.
Here's the deal. Our 6500A is not exactly a "new" drive. One of the primary reasons it's important to keep up with firmware updates is that as new media becomes avaliable the drive manufacturer has to add an entry to the media table in the drives ROM so that the drive will recognise the disk you insert and know what type and how fast it can be burned. Obviously a drive with older firmware won't know about media released 6 months or 1 year later, right?
It is highly likely that although Tesco probably isn't selling the highest quality media, it may just be new enough that the drive doesn't recognise it. As I said, my sister bought a batch and took them back. I've sent her a newer drive to replace one of her oldest drives (NEC 2510A) and we're going to flash the other one with updated firmware to bring it up to date, and hopefully solve the problem. The true test will be trying to use one of the Tesco disks on the newest drive to see if it's just a disk that wasn't in the media table of her oldest drive. There may not be anything wrong with the disks from Tesco at all.
Coincidentally, I flashed one of my 6500A drives with the latest firmware from Liggy and Dee's site yesterday and have had no problems with my laptop burner.
Check to see which version of the firmware you have in your 6500A. More specifically, what the prefix identifier is (2.XX or 4.XX) and make sure the latest firmware you download and flash to has the SAME PREFIX. I cannot overstress this.
So to recap:
Find the drive's current firmware (Use the My Computer>Properties>Hardware>Device Manager>DVD/CD-ROM drives> and select the Details tab for the 6500A to see what the FW version is)
Navigate to Liggy and Dee's 6500 page. (link above)
Find the latest firmware for your drive with the same prefix identifier (2.XX or 4.XX) and pick the 'flavor' you want. (Personnally I always use the RPC1 version so I can watch region 2 movies on my region 1 machine.) And download it to your desktop.
Find the link on the left side of the page for Binflash and select that to go to the Binflash download page and look for the Win32gui version of the flasher (currently 1.37). Download that.
Extract both zips to separate folders on your desktop, copy the Binflash utility executable to the other extracted folder containing the firmware file you downloaded and then doubleclick Binflash executable it to get started. It will identify your drive when it runs (make sure it shows the 6500 in the selection window) and then hit the Flash button. It will then open the file select window and will show the firmware you just downloaded. Select it and then flash away. Reboot when it's done. You can delete the files you downloaded and extracted when it reboots (assuming everything went as planned). Pop in a disk and try it out.
It is a shame though that drive manufacturers drop firmware update support after a while because they have released newer drives and want us to buy a newer version simply so we can read newer disks. But that's what keeps the wheels turning I guess.
Post back and let us know how you got on..