Read only dimensions?

What the hell is a read-only dimension?
I'm trying to use an equation in a top level assembly to tie together a dimension in one subassembly with an equivalent dimension in another
subassembly.
It tells me I can't do it because the dimension is read only.
On other occasions it won't even let me pick one of the dimensions to be driven by the equation, saying "Cannot select a dimension from another model to be driven by an equation in the current model". I've successfully done this within this top level assembly already, so why is it now giving me this error?
TIA John H
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A read-only dimensions is a dimension that cannot have it's value changed because something is driving it. For example, if the dimensions is driven by an equation, it will be set to read-only. On other words, you can't have two equations drive the same dimension. It has been years since I saw SW mishandle this, so some model somewhere has an equation driving this dimension.
The second issue sounds like you are editing a component in-context to the assembly. In that case, you will only be able to add equations that reference that component. If you are editing the assembly, then something else is going on.
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OK.....I've found that the "read only" box had become ticked in the properties of the relevant dimension.
I don't know why this happened, as I'm the only person to have worked on it. Could it have been something to do with copying an earlier assembly (and its sub-assys) and then opening each up and changing file references to point to the new files? At one point I was prompted as to whether I wanted the assembly equations to link to the new file, which I said "yes" to, but then next to the "read only dimension" tick box there was a reference to the original assembly. Any thoughts?
The other problem of "Cannot select a dimension from another model to be driven by an equation in the current model" still remains to be solved.
John H
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SW may have somehow set that (or failed to clear it) during that process. I have seen such things, but only rarely.
I don't have any more ideas on the other issue. Time to get the VAR on the line?
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