First make sure that the hard drive is formatted in FAT32 or Mac OS Extended (Journaled).
Windows NTFS or exFAT formats will not work here.
Open AirPort Utility, select the Time Capsule and click Manual Setup
Click the Disks icon at the top of the window
If your drive is recognized, it will appear here as confirmation
If you are sure that the drive is formatted in FAT32 or Mac OS Extended (Journaled)...also known as HFS+....and the drive does not appear in AirPort Utility, you may need to use a powered USB hub with the drive....even if the drive has its own power supply.... because the USB port on the Time Capsule is underpowered.
Apple Support Documents: