The 'other' stuff can be a pretty tricky pain in the ass. Spent a solid week, a few months ago, trying to figure out how to get rid of mine (it was like 90 gigs of a 500 gb drive) with more or less no luck. Was very frustrating
Yes it can. In our case it was FCP files buried deep in the drive. 2 files amounting to 320 GB. Got rid of both and reclaimed almost all of the 320.
Glad to see you've gotten this resolved...
Repeating the link to the set of lessons that explain FCPX media management in great detail. FCPX is unique in the way it handles files which can be very large via its library/event/project/backup/render/proxy etc. Paying for the full set of Ripple Training's media management lessons will be invaluable to her if she'll be doing lots of FCPX work.
http://www.rippletraining.com/products/final-cut-pro/final-cut-pro-x-media-management/Media management via an external external Thunderbolt or USB3 drive for HD or 4K video editing is important and keeping video/media content off the boot drive even if the boot drive is an SSD. A Thunderbolt RAID 5 array specifically for video editing is a good idea with FCPX as fast and large storage is desireable with FCPX.
Blackmagic Disk Speed Test is available for free in Apple's App Store is great to evaluate SSD or HDD drives for how they'll work for video editing.
https://itunes.apple.com/us/app/blackmagic-disk-speed-test/id425264550?mt=12