For most Tandy 1000 models (other than the compact EX and HX) that did not come already equipped with a hard drive, Tandy offered hard disk options in the form of hardcards that were installed in one of the computer's expansion slots and consisted of a controller and drive (typically a 3.5-inch MFM or RLL unit with a Western Digital controller) mounted together on a metal bracket. Their own 20 MB hard card was offered for $799, though compatible third-party units were available. Although this arrangement provided a neat physical coupling between the controller and the disk, single-sector internal transfers and dependence on the speed of the host machine to transfer data to memory meant that a trial-and-error approach was still needed to set the disk interleave correctly to ensure optimum transfer rates. Furthermore, as the Tandy 1000's slots were only 10.5" long and are 8-bit only, some units would not fit and/or operate correctly unless they were certified to be Tandy-compatible.