Arguably, "figured it out myself" is not a very helpful answer to a situation like this, as people looking for answers reach a dead end. Anyways, if your disk happens to be ExFAT, and you remember improperly removing the disk the last time you used it, chances are, fsck is holding it hostage (metaphorically, of course). fsck is trying to repair the disk, but it will stall forever, so you want to launch Disk Utility, and the Terminal. Try to mount the disk, and switch to Terminal and write sudo pkill -f fsck. The disk will mount in read-only mode, and from there, you can repair it with First Aid in Disk Utility. This won't work for non-ExFat drives, as macOS manages them differently.

partially solved by compiling with Xcode 10, not the best way to go about it, but i don't plan to upload my app to the app store. also seems to bring most features from iOS 12 into 13, like the classic 3d touch, and outlined switches. delighted with the result!