IBM 5150/5155/5160 - Using an External 3.5" Diskette Drive
Assumption
Your IBM 5150, IBM 5155, or IBM 5160, has the IBM supplied floppy controller fitted.
IBM refers to that controller as the IBM 5.25" Diskette Drive Adapter.
A photo of that controller is at here.
Scope of this web page
1. The physical connection of a 'standard' 3.5" diskette drive (either 720K or 1.44M) to the external connector (37-pin) of the IBM 5.25" Diskette Drive Adapter.
2. DOS configuration that enables DOS to read/write 720K sized 3.5" diskettes in that drive.
Method summary
If we were to simply connect the 3.5" diskette drive, then change the 'floppy drive count' switches on the 5150/5155/5160 motherboard, DOS would think that a 180K or 360K floppy drive is what was connected.
So, instead, we will connect the 3.5" diskette drive, then use DRIVER.SYS with "/t:80 /s:9" parameters so that DOS knows that the drive is a 720K one.
Diagram
A related diagram is at here.
Restrictions
1. Even if the 3.5" diskette drive is a 1.44M one, 1.44M sized diskettes cannot be used. That is because the IBM 5.25" Diskette Drive Adapter cannot operate at High Density (HD). You are restricted to using 720K sized 3.5" diskettes.
2. It is not possible to boot from the 'external' diskette drive. (Because the first internal floppy/diskette drive [physical 0] is the only floppy/diskette drive that the IBM BIOS uses as a possible boot device.)
3. The IBM 5.25" Diskette Drive Adapter does not supply drive power on its external connector. You need to use a 'standard' 3.5" drive and power it via its dedicated power connector.
Notes
1. This procedure has no effect whatsoever on any fitted 'internal' floppy/diskette drives; you do not need to remove nor adjust them.
2. In the steps that follow, I do not (repeat: not) have you change the the 'floppy count' switches on the motherboard. For this procedure, where we use DRIVER.SYS, there is no reason to change the switches. Also, setting a count of 3 or 4 can cause a problem for XT-IDE cards.
3. Below, in step 3, I have DRIVER.SYS located on the hard drive. If you have no hard drive, there is no reason why you cannot have DRIVER.SYS located on a DOS boot floppy for use in drive A: (the first internal floppy/diskette drive [physical 0]).
STEP 1 - CONSTRUCT DATA CABLE
You need to construct the required data cable.
The photos and diagrams at here and here provide you with enough information to do that.
I simply found one of my many spare PC floppy cables, cut off its controller connector, then fitted the 37-pin controller connector (observing where the gap needs to be).
STEP 2 - CONNECT 3.5" DRIVE
1. Connect the 37-pin connector of the cable to the 37-pin connector of the IBM 5.25" Diskette Drive Adapter.
2. Connect the 3.5" drive to the last connector on the data cable. In the photo at here, that connector is the yellow-triangle highlighted one on the left.
3. Connect the 3.5" drive to power.
STEP 3 - CONFIGURE DOS
Assumption: The DOS directory on your hard drive is in the root, is named "DOS", and contains the DRIVER.SYS file.
1. Use whatever means you have to add the following line to the CONFIG.SYS file on your hard drive:
DEVICE=C:\DOS\DRIVER.SYS /d:2 /t:80 /s:9
STEP 4 - REBOOT
Reboot your computer (from the hard drive).
DRIVER.SYS will display a line that looks a lot like, "Loaded External Disk Driver for Drive D".
The drive letter (drive letters are logical drives) shown in that line, is what you use to access the external diskette drive.