I've reinstalled Mac OS High Sierra on my 2017 Macbook Pro. I have my raw files on an external Lacie drive and also have the catalog on the same drive. Nothing is stored on the internal drive apart from LR itself.
I just imported my photos (about 11000 of them) and asked them to stay at the same location, ie my external drive. However, it looks like since doing this, about 60gb of my internal drive has now been used.
Given my catalog and all my photos are on another drive do you know why this would be? There are also no other programs installed on my laptop yet, and it's not connected online, so it wouldn't be other programs or updates causing it.
I have seen High Sierra taking up a lot of extra space as well sometimes. These could be temporary (cache) files, for example. And Time Machine can make a temporary backup file if the Time Machine disk is not available. It could also be a glitch in Apple’s new AFPS disk formatting, because like you I have seen it on my MacBook Pro with SSD disk formatted in AFPS, but not on my iMac with its HFS+ formatted disk. After a while the space comes back.