What is an MFM-harddrive? I know it's in old computers, but is it something special? Or was it used before IDE there was? Has it special cables?

From; http://www.datarecoveryspecialist.com/datarecoveryspecialist/corporate/terms.html DISK DRIVE RECORDING: DATA ENCODING SCHEMES FM ENCODING (Frequency Modulation Encoding) - This is an outdated encoding scheme that was used in 8 inch floppy disk drives during the 1970's. The problem with it was that it used up half of the disk space for timing signals used in the encoding process. Later (in the 1980's) the technology was refined and replaced with a new standard called MFM encoding. MFM ENCODING (Modified Frequency Modulation Encoding) - This is an encoding scheme was was an enhancement to FM. Basically this scheme converts the digital bits from the computer into a pattern of magnetic changes or "flux reversals" that are stored on the hard drive. MFM does away with the need for timing signals thus it is more efficient than FM. It was widely used in hard drives in the 1980's and is still in use today in floppy drives. RLL ENCODING (Run Length Limited) - This is an encoding scheme that reduces the amount of data-checking information that is stored and thus requires less flux reversals for a given amount of data. The logic circuitry is more more complicated than MFM but allows much more data to be recorded on the disk drive. In RLL 2,7 the "run length" of zeros is limited to 7. The codes are chosen so that the sequences of zeros in the codes always range from 2 to 7. This allows for a 50% increase in disk space over MFM. ARLL ENCODING (Advanced Run Length Limited) - This is an advanced version of RLL that has a run length of zeros from 3 to 9 which even further increases disk space up to (100% over MFM).

Yep, the old twin-cabled hard drives - 1 data cable & 1 control cable. Not too reliable, but that's all there was at the time. Generally ran from their own specific HDC (Hard Drive Controller) card, predated CMOS, but could be 'fudged' to work in newer machines. Not much capacity, most were around 32 Mb (or less) - the old DOS3 limit.

This is going back to steam computing. When the IBM-PC first came on the scene you could actually have it running with one or two floppy drives and no (repeat no) hard drives. If you decided to have a hard drive, being early days there were various standards such as IDE, MFM and RLL etc., just as there were in early videos and music cartridges or cassettes. In order to connect these hard drives, you had to purchase a compatible controller card or multi-functio i/o card. From memory they used the same cables. Hope this helps.

"steam computing" ;>)