Just thought I'd let you know that since doing the update to OSX 10.5.5 I've had numerous bad frame errors and disk I/O errors in M100 12.7 I think others have reported similar problems. These were mostly related to old M100i media.
Fortunately, I keep another internal boot drive that is always one update behind and after cloning it back to my primary drive all seems well. I also needed to repair my G-Tech Gspeed raid with Diskwarrior as it was not mounting reliably- maybe related to the update maybe not?
Would be nice to know what the problems are - QT related or deeper than that? Rolling back to QT 7.5 as described by Floh and others did not cure my issues without going back to 10.5.4
I guess we need a M100 update before moving to 10.5.5
We haven't seen any of the problems you're describing related to OS 10.5.5 but I must admit we are full into v13 testing right now.
As this QT 7.5.5 thing has been playing out, I've talked to a few users for whom the "fix" got botched somehow and they had to try again. "Get Info" on your QT Player app to ensure that it is, indeed, reporting as v7.5. If it still says v7.5.5, the something didn't take right when you tried to back up. If that's the case, take a look at my recent post below for the most complete set of steps and links to date.