What you are asking is not possible - you can't meaningfully lock a USB drive without encrypting it, as plugging it into another device will make it accessible.
Bitlocker is probably the best way to encrypt a drive for a Windows system, otherwise something like Veracrypt could work as well - but both these utilities are going to take a considerable amount of time to lock the drive.
Depending on how full your drive is, you may be able to greatly reduce the amount of time bitlocker takes to encrypt it by turning on "Encrypt with used disk space only" - this will only encrypt the data in use on the drive, which means if you are only using 70 gigs it will take 1-4 hours.