My follower is Borghak the Steelheart. Her inventory has great stuff and I want her to hold it while we part ways so that I can try other followers. Will she hold on to it, or will it disappear mysteriously?

No she will still have it.

She will have it, unless she dies of course.

Dangerous leaving your best stuff with a Follower. i wouldn`t do it even if they do hold on to it. Anything could happen to them and *poof* gone!

Before I brought the Whiterun house I used to leave my things with Lydia... after all, she is swoooorn to carry my burdens.