rekordbox writes error writing to usb device

Hello.

I have a problem with rekordbox.
My pendrive was full, and I tried to make some space on it. I found out, that there are still musics from old playlists on it, so i wanted to delete all of them.
Where can I find, which musics aren't in any playlist in rekordbox?
I needed to select and delete them manually. But during deleting after a while, rekordbox always crashed. I started with only selecting 200-300 files, and successfully deleted them. I wanted rekordbox to update collection on my pendrive so i ejected it. But it wrote Error writing to usb device (or something like that).
I said okay. I reformatted my whole pendrive (to FAT32). I just copied 30 GB of files with total commander, it worked fine. I deleted them, and started rekordbox, and copied back my whole collection again. It wrote that error again. I tried reformating my pendrive with slow formatting to FAT32. I copied only one playlist to it, and it was successful, but when i want to move the whole library, it always writes that error. What can i do to make rekordbox copy my library to my pendrive successfully?