Please insert a disk into Disk Drive (D:)

Hello!

A friend of mine was moving some files into a USB flash drive and, after the operation was completed he used the Safely Remove Hardware and Eject Media to check if it was safe to remove the drive. After he confirmed that it was safe to do so he removed the drive just to receive a message right after telling him that the drive was still in use by a program, so he reconnected the drive again, and since then the drive is indicated by a greyed out icon and it became completely unusable, giving a 'Please insert a disk into Disk Drive (D:)'. We tried different ports, uninstalling and reinstalling the drive, and some other solutions but there is no outcome.