I don't like multi-session recordings. If they go bad, they really go bad. On a single session disk the Catalog track is written on the inside. With multi-session burns the primary catalog track still exists, but it also points to additional information located else ware on the disk. All I can think is the following. 1.) bad burn, 2.) temper glitch (try rebooting), 3.) You used some non-standard burning method that isn't Mac or pc compatible- not likely. 4.) Drive issues, perhaps with calibration.
Try it on another computer. Try loading another disk in and seeing if it can be read. Try burning another disk to see if it is the drive or the media. Let the disk and the drive cool off and see if that helps (a long shot)
CD and DVD drives use stepper motors. Stepper motors (mechanisms that move in integer steps, not fractions of steps) are not used in hard drives because thermal expansion caused alignment issues. These drives would self calibrate, but sometimes the difference between the platter expansion (most significant) a the expansion of the stepper motor screw (least significant) was greater than the drive could compensate for. Any way, optical media does expand and most optical drives use stepper motors. It is at least conceivable that thermal expansion could play a roll in why some disks wont mount sometimes.
Dust, thats another possibility. Hopefully it isnt the drive failing.
In the future I suggest the following: Don't use cheap media. Practice good media handling techniques. Try not to burn in multiple sessions.
I like your handle; its a good name.
Best of skill and luck in life.
Post Script: My bias against multi-session burning comes from problems that I have with the burners at my work. I make a lot of DVDs and CDs, and when burn fails it just makes the day longer. I had a lot of failed multi-session burns at work, so I don't use them.