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I've seen a few computers with more than two floppy drives, most with two 3.5" and one 5.25". My question is how was this achieved? I've heard that it could be done with a second controller card, but I'm not aware about FDD controller cards for PCI.
Also, what about those four-connector floppy cables (one edge connector, three standard goldpin connectors) - could've they been used to connect a 5.25" and two 3.5" drives, how about one of iOmega tape drives?
A PC floppy controller can control two drives. For the third and fourth drive you'd need a secondary controller. I haven't ever seen one for PCI, only for ISA - some ISA cards can be configured to primary or secondary.
Edit: Raffzahn is correct: early floppy controllers did support all four drives the Shugart bus provided for. This changed with the clone market and when 3.5" disks came up (AFAIR).
The four-connector floppy cables were used in the 5.25" to 3.5" transition era - they simply provide one kind of connector each for the A: and for the B: drive, so you can connect any mix. You can only use one of the connectors in each position.
Edit2: If you don't stick to the twisted-cable scheme and jumper the address on the disk drive you can actually use all positions simultaneously (with a controller capable of driving four drives and if space allows).
Edit3: As it appears, I've looked at a non-common controller - most four-drive controller seem to use two cables with two drives each. (Thx @Raffzahn)
If this wasn't Retrocomputing I'd say get a USB floppy drive...
Following the Shugart standard, selections for 4 drives (DS0..DS3) are available on a single interface. The drives had to be jumpered accordingly. Hardware-wise the PC floppy controller also supports this. But as the original housing only allowed two (full height) drives, the controller thus split the interface onto two (proprietary) connectors: one PCB header for up to two internal drives, and a DB37 for two external drives (separate housing). Each features only two DS signals (DS0/1 internal, DS2/3 external).
To simplify installation, IBM invented the twisted floppy cable, swapping the two select lines between the PCB connectors for the drives. This took away the need for individual jumpered drives. IBM drives were always set to DS1 (*1), and addressing was done via the connector used. In fact, there's another (logical) twist, as the outermost connector is always the 'first' drive. This is due to the termination needed / built in (*2) on (early) 5.25 inch drives.
Nice design for simplifying upgrades, but it complicates more sophisticated installations - like 4 half height drives within the original case.
With an original controller it's no problem to have 4 drives. Just cable work.
Clones with integrated/on-board controller often had two pin headers for floppies, so 4 internal drives could be used (*3). Later again the second connector got often dropped, so more recent machines only supported two drives. You may want to check the manual for your machine.
Also, with 3.5 inch drives becoming standard, clone manufacturers invented the 4 connector type cables you mentioned. These are not meant to support 4 drives, but rather offer the choice to have either for each position. Usually (*4) the connectors are paired in a way making it physically impossible (*5) to add more than one drive for each position.
Edit: Here is a nice page showing the development in detail.
*1 - It's not that easy, since IBM did use drives with different labeled selector blocks. With the ones labeled DS0..DS4, DS1 is the right selection, while on drives with a DS1..DS4 labeling, DS2 needs to be selected.
*2 - Termination is another issue to take care of, at least for 5.25 inch drives. Only the last (A) drive needs one, while the second must not have it. It's a resistor array, usually looking like a socketed IC often of yellow/brownish colour, but sometimes also like a SIL package. 3.5 inch drives do not need to be changed here, as their behaviour is like automatic. It can be added as A or B without changing anything.
*3 - Be careful, some had two pin headers, but each only supporting one drive - basically moving the cable twist onto the motherboard to save on cabling costs by only needing to supply two straight cables with single connectors (and no 5.25 support).
*4 - As so often with Taiwanese creativity, there are cables made without that restriction in mind.
*5 - Well, almost - never underestimate users with a dedication to add a third drive
You'd definitely be looking for an ISA controller; it seems highly unlikely that anybody would make a PCI one as there would be little commercial application, and developing PCI cards is rare for hobbyists.
There are plenty of ISA cards available, however; this one is an open hardware design, so you can produce your own if you have the equipment to make PCBs. (It's also worth looking at the designer's list of projects if you have any other similar requirements: he's produced an entire modular PC-XT compatible system based on an ISA backplane, so some of those boards could be very handy.)
In addition to the other answers, SCSI floppy drives do exist, although they are fairly uncommon. These would be usable on even modern systems, as you can get SCSI controller cards for nearly all generations of PC clones.
We used to sell 3-drive Apple-II systems for bookkeeping.
One drive for the Accounts, one for the Transactions and the third for the Stock/inventory.
We would simply add another floppy disk controller.
