Converting old movie files

This is a hugely complex question because there are lots of variables. For what purpose are you doing this? Movies have two components, audio and video. Sometimes there are multiple audio tracks. For each of those components you have different forms of encoding. This can mean that "converting" just requires a simple repackaging of the existing components, or it can mean you truly need to convert the components to another format. Then there's the packaging to which you wish to convert. AVI is an old wrapper. Converting components can mean quality loss and is best avoided. Sometimes converting from AVI simply involves putting the same components in a new wrapper. Other times you need to convert one or multiple components as well as re-wrapping. Now all onto all that the fact that there are multiple packaging formats. MP4 is common, but there are others. Now consider that not all software will play all components or be able to handle all wrappers.

So, what are you doing?

Some people may tell you to open the file in a program such as Handbrake and convert it to MP4. Handbrake has a large number of setting and conversion formats. If your file already uses mp4 components then there would be no point in truly converting them unless you want quality loss. So even a simple instruction like that -to be done properly- needs familiarity with the file details.

I'm puzzled by that message. Do you have Paragon installed? Macs should be able to read NTFS even if they can't write to it (my information may be out of date). On my Mac running an older Mac OS I just plugged a drive from a Windows computer into my computer and it read it without any issue just using Apple's software. It may be the very software that is supposed to help you read these drives is what is causing problems.

If these are AVIs that are several years old they may need to be re-coded (which is a pity since there will be quality loss). That particularly goes if you plan on using these files in iMovie. My understanding is Apple recently dropped support for older video codecs. Those are what are used to encode the picture element on movies. You have to convert them to a newer format. It also was common for AVI to use mp3 audio which is not MP4 compliant.