Western Digital Gold or Black disks have a long warranty and seem to be reliable. 4 TB or 6 TB disks should be adequate. Use four drives at a minimum; six if you anticipate very heavy work loads. Unless you routinely generate samples in excess of 1 TB, then likely about 12 TB of storage is adequate. Figure that each sample will transiently occupy about 4 times the final stitched data size: raw data, plus compressed raw data, plus original stitched images, plus cropped stitched image. Perhaps you keep 10 to 20 samples on there at any one time whilst they await processing and transfer to the server. On a multi-user system, disk usage will always expand to fill the available space. It can become a headache to manage a large server with dozens of samples (especially if there is a disk failure), so err on the side of less space.