Pioneer PD-M550 upon play it cycles each disk (6) in and out and display says "diSK" after each disc

Any way you can.

The last 3 discs might be CD-RW which can be rewritten, but not all cd players will be able to read them. Just go to the store and buy some CD-R disks and re-burn the last 3 disks. That should solve your problem.

To my surprise, CD cleaner disc did it for me. It's a very old Philips one with a bit of furry stuff on the track side. I was amazed it even played, but it did and after letting it run for 20 secs, tried proper discs and it works fine. And this Pioneer PD-M603 is at least 15 years old!Looks like it would be hard to clean the lens manually because it's a long way inside.

Hi. You need to replace the opu (optical pickup unt), laser in other words. The laser diode is slowly getting weaker and the lens works in conjuntion with the laser. The reason it works when it's upside down is because gravity is pulling the lens closer to the disc, thus it does not need as much power to focus onto the tracks etched into the disc. Replace opu with type PEA1030 aka PU-23 http://www.donberg.co.uk/catalogue/laser_units_cd_dvd/laser_pu_23.html. Hope this helps you out and good luck.

CDs go in the Upside Down position. Your PD M501 will accept PRW 1139, PRW 1141 and 163.
Lexus Models: CDX-M9071zt CDX-M8036, CDX-M9071, CDX-M9741, CDX-M9751
also:
Philips CDC-875 Pioneer CDX-FM35 Pioneer CDX-FM45 Pioneer CDX-M30 Pioneer CDX-M40 Pioneer PD-DM802

* Pioneer PD-DM902 Pioneer PD-M403 Pioneer PD-M423 Pioneer PD-M426 Pioneer PD-M450 PD-M453 PD-M455
* Pioneer PD-M501 Pioneer PD-M502 PD-M503 Pioneer PD-M510 Pioneer PD-M530 Pioneer PD-M550 Pioneer PD-M552 Pioneer
* PD-M600 PD-M601 PD-M603 Pioneer PD-M610 Pioneer PD-M630 Pioneer PD-M650
* Pioneer PD-M701 Pioneer PD-M702 Pioneer PD-M703 Pioneer PD-M710 Pioneer PD-M730
* Pioneer PD-M801 Pioneer PD-TM1 Pioneer PD-TM2 Pioneer PD-TM3 Pioneer CCS-430
* DCM-444 and DCM 777

BMW 740 OEM

Sometimes the little ball bearings that keep the cart trays separated come out causing tray jams.
Make sure your tray retracts all the way into the player flush with the face plate, gently push it in while retracting for those occasional no-read situations. (Also for Denon that takes these same carts). Also note that the CDs go in the Upside Down position.
Applicable to:
Lexus Models: CDX-M9071zt CDX-M8036, CDX-M9071, CDX-M9741, CDX-M9751
also:
Philips CDC-875 Pioneer CDX-FM35 Pioneer CDX-FM45 Pioneer CDX-M30 Pioneer CDX-M40 Pioneer PD-DM802

* Pioneer PD-DM902 Pioneer PD-M403 Pioneer PD-M423 Pioneer PD-M426 Pioneer PD-M450 PD-M453 PD-M455
* Pioneer PD-M501 Pioneer PD-M502 PD-M503 Pioneer PD-M510 Pioneer PD-M530 Pioneer PD-M550 Pioneer PD-M552 Pioneer
* PD-M600 PD-M601 PD-M603 Pioneer PD-M610 Pioneer PD-M630 Pioneer PD-M650
* Pioneer PD-M701 Pioneer PD-M702 Pioneer PD-M703 Pioneer PD-M710 Pioneer PD-M730
* Pioneer PD-M801 Pioneer PD-TM1 Pioneer PD-TM2 Pioneer PD-TM3 Pioneer CCS-430
* DCM-444 and DCM 777

BMW 740 OEM and others.

the lens on the optical pickup is dirty and needs cleaning. they sell a lens cleaner disk at any electronics store. you also might want to blow any dust out of the unit with canned air to prevent it from happening again.

MOST LIKELY IT SOUNDS LIKE THE MOTOR THAT SPINS THE DISC - NEEDS REPLACING

turn the cd, the silver side must be look down