Quicktime 7.4 will break Metro the 'save' functionality of all versons of Metro prior to 6.4.1.1 resulting in an error -54 message.
Do not install Quicktime 7.4 if you want to continue to use Metro, Metro LX or Metro SE before 6.4.1.1.
Metro 6.4.1.1 fixes this problem allowing save to work. However this problem is within Quicktime 7.4 and thus could introduce other problems.
Perhaps Quicktime 7.4.1 will fix these issues.
Edit: Indeed, Apple's release of Quicktime 7.4.1 fixes this issue!