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The first IBM XT computers used MFM hard drives. MFM allows a transfer rate up to 5MB/s and were only available in sizes from 5MB to 110MB. The data cable was 20 pin and another 34 pin cable. These drives fit into a 5.25” bay. It was not uncommon to have as many as 10 or more bad sectors with the drive still fully functional despite the areas that were marked. The drives had to be parked before the PC was transported to prevent damage from the drive heads impacting the disks.The first IBM XT computers used MFM hard drives. MFM allows a transfer rate up to 5MB/s and were only available in sizes from 5MB to 110MB. MFM uses a twin-cable hard drive interface. The data cable was 20 pin and another 34 pin cable. These drives fit into a 5.25” bay. Almost all MFM drives had 17 sectors per track. It was not uncommon to have as many as 10 or more bad sectors with the drive still fully functional despite the areas that were marked. The drives had to be parked before the PC was transported to prevent damage from the drive heads impacting the disks.The successor to the MFM drive, externally appearing the same as an MFM drive. This encoding method can store 50% more data on the disk than MFM. The RLL drive connected to the RLL controller with two cables just likes with MFM. The RLL encoding method is still used today although RLL drives are not.The successor to the MFM drive, externally appearing the same as an MFM drive. This encoding method can store 50% more data on the disk than MFM. The RLL drive connected to the RLL controller with two cables just likes with MFM. The RLL encoding method was still used on IDE drives although RLL drives are not. Until the mid 1990's all IDE drives used RLL encoding. If a 10MB MFM drive formatted with a RLL controller could store an additional 5MB (making it 15MB) and perform 50% faster. RLL drives put 26 or 32 sectors per track, unlike the MFM 17 sector per track formatting. However, the early practice of retrofitting RLL controllers to MFM drive systems resulted in reliability problems. RLE is not a type of drive, but a way to format a drive. Whether or not the drive becomes MFM or RLE is determined when it is low level formatted and attached to either type of controller. Early MFM drives would often fail when formatted to RLE, until later RLE certified drives were introduced. You could get more out of your MFM drive by attaching to an RLE controller, but you risked reliability if your MFM drive was not RLE certified. == EDSI ==*'''Enhanced Small-Device Interface''' A somewhat obscure enterprise level drive technology dating back to 1983. Maxtor company introduce drive technology for extreme high end systems. EDSI was considered a direct competitor to the later dominate SCSI drive technology.IDE drives use an embedded controller, unlike MFM/RLE. When MFM/RLE the controller was on the controller card. It is somewhat of a misnomer when the term IDE controller was introduced, since the controller has always been built into the IDE drive, so the card is actually just a host adapter. Prior to Pentium and laster 486 systems, a host adapter card (referred to as the IDE controller card) was placed in one of the card slots on the motherboard. The IDE host adapter (controller) became standard integration onto the main board post 486 systems. PRML (partial result maximum likelyhood) succeeded RLE encoding for IDE drives in the mid 1990's. This is part of the new self monitoring S.M.A.R.T technology introduced in 1995 and implemented on all IDE, SATA, SSD and SCSI hard drives. SMART data is accumulated and stored inside a hard drive to evaluate the performance and history of the drive. Problems and events stored on the SMART drive can be displayed using many different software utilities. PRML is Seagate’s proprietary method for statistical analysis of the data.