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Re: Locked files on USB sticks in Linux Mint 19.3

I'll just have to make sure I don't delete files that aren't on the PC.

I do hope you don't mean that!  I hope you mean "I'll just have to make sure I don't delete files that are on the PC" !! Please confirm that. 
As for Root:  It matters not, but you might feel more comfortable closing the file manager and re-starting it. 

Let me know when you want to delete the USB files - just pressing "Delete" is  not enough!
I'll just have to make sure I don't delete files that aren't on the PC.

I do hope you don't mean that!  I hope you mean "I'll just have to make sure I don't delete files that are on the PC" !! Please confirm that. 
As for Root:  It matters not, but you might feel more comfortable closing the file manager and re-starting it. 

Let me know when you want to delete the USB files - just pressing "Delete" is  not enough!