Duplicating 9940 to LTO4

I assume you mean you have a 6.5 media server attached to the 9940 and a 7.6 media server attached to the LTO4 library?

You should be OK but obviously there is no support for anything 6.5 these days.

To be as fast as possible you would match the number of LTO4 drives with 9940 drives so that vault can do its stuff

Maybe for the period of doing this exercise you may want to allow different retentions on tape so that all the different retention periods can go onto a single set of tapes.

You could also restrict the maximum number of partially full media in the target pool you are using to make sure that it uses tapes as efficiently as possible

As for how long it will take ... to be honest i wouldn't even like to guess .. you could do the maths if nothing else is likely to run during the whole process but even that is likely to not be too accurate!

Hope this helps