Importing music in to RB playlists means that you assign a filepath, in this case, from USB stick to RB. You don't copy the music files in to RB. When you remove the USB stick, RB can not find the file source anymore, and all your music files instantly become 'corrupted' as you wrote.
You can copy-paste your USB stick to the MB drive, and change the filepath from USB to MB drive in RB settings.
In my case I keep all music files on an external 1TB Lightning SSD, formatted in FAT32. My MBP is just for RB and Serato software. It's a more safe way of handling your precious songs I think. And every now and then you can backup the external SSD for extra safety.