For example, if you want to buy an old floppy disk drive on ebay, always look for this resistor on the offer images. Often this is missing because the drive was used as the second drive. Here the resistor must be removed! Further information can also be found at Herb Johnson: Resistor termination for floppy data cables
Resistors with 150 ohms, as they are used in many old floppy disk drives, are now virtually no longer to buy. Below are two important product descriptions: Resistors Fa. Beckmann / Resistors Fa. Vishay (Dale)
Please pay attention, there are these resistors with 2x7 = 14 pins and with 2x8 = 16 pins! In addition, there are also floppys with separate resistors per line, e.g. with 220 + 330 ohms. In case of doubt always look into the documentation.
If a drive suddenly behaves strangely and makes strange sounds after a modification or expansion, check if the resistor is plugged in correctly. With two drives, only the last drive on the floppy cable has a resistor!