Always use a locally mounted Mac OS X Extended formatted volume to store your Aperture library. Mac OS X Extended is the only file system supported by Aperture.
If you store your Aperture library on a volume that is formatted FAT32 (a file system commonly used on Windows PCs), you may encounter issues including poor performance, data corruption, and data loss. Referenced images stored on FAT32 volumes may sporadically go offline. Many external drives come pre-formatted as FAT32. Use Disk Utility to format new external hard drives as Mac OS X Extended before using them with Aperture. For instructions on how to do so, see the steps in Format external drives to Mac OS Extended before using with Aperture.
Also, it is strongly recommended that the Aperture library be stored on a locally mounted hard drive. Storing the Aperture library on a network share can also lead to poor performance, data corruption, or data loss.