User permissions on volume drive
Try going into Terminal then sudo chmod 777 /Volumes/ExternalDriveName this should allow everyone access. If it doesn't work, then verify that it is formatted APFS, HFS, FAT32, or EXFAT. If it is formatted as NTFS, then it will be mounted as read only. If it is formatted as something that macOS can write to, then try running First Aid on the external disk. This might fix permissions. Good luck!