Depending on the year, I believe them to be WD GREEN drives... the lowest performance of a reasonably good line-up.
That's what mine was when I replaced it with a larger capacity drive.
Here's from WiKi:
The hard drive typically found in a Time Capsule is the Hitachi Deskstar, which is sold by Hitachi as a consumer-grade product (the Hitachi Ultrastar is the enterprise version). Apple labeled the drive as a server-grade drive in promotional material for Time Capsule, and also used this type of drive in its discontinued Xserve servers. Apple states that the Hitachi Deskstar meets or exceeds the 1 million hour mean time between failures (MTBF) recommendation for server-grade hard drives.
The 500GB, 1st generation Time Capsule shipped with a Seagate Barracuda ES-series drive.
More recently, other hard drives such as the Western Digital Caviar Green series, and the Samsung EcoGreen series have been reported.