John Lukach
AirPort Utility 5.6.1 for Windows
The AirPort Utility for Windows allows Microsoft computers using Bonjour to access the Apple Time Capsule hard disk. The drive is available as a network share through UNC mapping on your PC. The binary data stored in HKEY_Users\S-1-5-1234567890-1234567890-123456789-1000\Software\AppleInc.\Preferences\com.apple.airport.diskagent will provide confirmation of which volume is associated with your Apple Time Capsule. An external USB connection is available so you could have two volumes listed.
If the end-user setup Windows Backups than you will be able to gain additional insight into the size of the disk with the free space available that may be beneficial in identifying the external USB drive.
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\WindowsBackup\ScheduledParams
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\WindowsBackup\ScheduledParams\Rules\
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\WindowsBackup\ScheduledParams\PresentableName
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\WindowsBackup\ScheduledParams\UniqueName
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\WindowsBackup\TargetDevices
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\WindowsBackup\UserDataExclusions
User defined inclusions are listed as numbered keys under the Rules folder containing specific paths.