An AV drive is a hard disk drive which is designed specifically for audio and video purposes. They were used in the late 1990s for sustained media transfers: they delayed thermal recalibration so it would not interfere with the transfer.
Also they were popular in the early ages of CD burning. Early CD writers ruined the disk when a buffer underrun occurred, which could happen when the hard drive would calibrate.
They are now obsolete, as normal hard drives are designed to handle realtime media through servo calibration to monitor head alignment and through the use of larger memory buffers.
References
Av drive Wikipedia(Text) CC BY-SA