Short answer is yes, external storage will solve this problem. This will standardize the location where the media are stored (typically, in the same directory as the database). However, it will mean that users can access the media only through the FileMaker database.
If that doesn't work for you, another solution will be needed. One solution would be, as you suggest, a standardized pathname stored in a field and an Open URL script step. (I have a colleague who uses this quite effectively.) However, you'll need to control access to the location where the resources are stored for this to work properly. If any Tom, Dick, or Harry can add (or, especially, remove) assets from the server, then the database won't be able to find them properly.
Another possible solution would be to use a third-party product called SuperContainer, from 360Works. This product does have a few advantages over your typical container fields (like the assets being more easily accessed from a web application, and working within Instant Web Publishing), but again, in order for it to work properly, the resources need to be added and removed via the FileMaker interface.
Basically, if you want this to work properly, you'll need to do two things:
1) Restrict access to the directory (or directories) where the assets are stored. Users shouldn't be able to add or remove material through the OS, but only through the FileMaker interface (or another way you control).
2) Break away from path name dependencies by allowing FileMaker to control the link to the asset, whether that be by intrinsic methods (external container storage, SuperContainer) or through your own efforts (inserting URLs into fields).
Thanks for your reply. I'm glad that the open URL method is OK. I think I will go with that one. I understand about controlling access to the server where all these files are stored. Various people put the data onto the server. We will just have to work out a standard file structure and stick to it. I think things work out better in the long run like that in anycase.
Use the new powers of v12 container fields to solve your problems.