I think it is a Toast problem, your version is old - even for OS 9.1. You really should upgrade to 9.2, it's free and a little more stable.
I would first consider moving up to a later version of Toast, v5 or v6, and trying that. I note you say "formatted" when you refer to the disks you made on the 7500. There should be no formatting involved. It may have something to do with sessions. A lot of CDROM's when used with the Mac, be they burners or just players, do not like to deal with sessions. It is normal, on the Mac, to close each session and to finalize the disk.
Do the disks that you burn on the 7500, mount on the 7500? Not all burners are created equal and yours may have a weak laser by now. I'm guessing it's SCSI @ 8X (or thereabouts) but I could be wrong. Of course, it could be that your G4 AGP has the weak laser. It could also have something to do with the brand of disks that you use. I have a CDR that will NOT burn Maxell disks at all. It will also not READ and disk that has a gold surface. It burns and reads, Office Depot $20 per hundred, like a champ.
If moving up does not fix the problem, then I would consider a thumbdrive, 256MB would be good-512MB would be better, to transport your files around. At 512MB, you are very close to the max a CD will hold, around 640MB.
To recap.
1. Upgrade Toast.
2. Change CD blanks.
3. Consider New Burner for both machines.
4. Firewire card for 7500 to run #3.
5. Thumbdrive when all else fails.
6. Don't use CD-R/W. Buy cheap CD-R and use them once. 20c each

Hope some of this helps