6.5 Working with FDDs

Keep the following issues in mind when working with FDDs:


3.5-inch HD FDDs are $10 commodity items made by several manufacturers. We prefer the Teac FD235HF, but there is little difference between brands. Buy whatever is offered. 5.25-inch FDDs are still available, but that won't last forever. If you need a 5.25-inch FDD to rescue data on old disks, get the drive now. If you patronize a local computer store, check there first. They may have a stack of old 5.25-inch FDDs they'd be happy to give away or sell cheaply.


5.25-inch FDDs require a 5.25-inch half-height, externally accessible drive bay. 3.5-inch FDDs can be installed in an externally accessible 3.5-inch third-height bay, or, by using an adapter, in a 5.25-inch half-height bay. The BIOS automatically detects installed FDDs, but can determine type unambiguously only for 5.25-inch HD (1.2 MB) FDDs, which spin at 360 RPM rather than 300. For other drive types, older machines assume 360 KB or 720 KB and newer systems assume 1.44 MB. Use BIOS Setup to confirm that the drive type is configured correctly.


If you add or replace an FDD, also replace the cable, particularly if it is the original cable. Manufacturers often fold and crimp the FDD cable for improved cable routing and airflow. An old cable that has been so treated is no longer reliable, especially after you disturb it to install the new drive.

Change Line Support

All but the oldest FDD controllers use line 34 for Change Line Support. When the FDD door is opened, the FDD signals on line 34 to tell the system that the diskette may have been changed. If you install a 360 KB or 720 KB FDD, verify the Change Line setting, ordinarily set on the drive by a jumper labeled "Line 34" or "Change Line." If you install such a drive in a PC/XT-class system, leave the jumper open. On any later system, install a jumper block to connect line 34 and enable Change Line Support. Failing to do so and then writing to a diskette in that drive may destroy the data on that diskette by overwriting the FATs and root directory entries with data from the diskette that was formerly in the drive.


FDDs are used so little nowadays?an occasional boot or program install?that head wear and media accumulation aren't problematic. Dust needs to be removed periodically. You can buy special FDD cleaning kits, but we don't bother with them. Every few months (or when we open the case for other reasons) we vacuum out the drive and drench it down with Zero Residue Cleaner or rubbing alcohol.


Don't. Replace the drive. Modern 3.5-inch drives are so cheap that it makes no sense to repair one, and they are often sealed units without repair access anyway. Older 5.25-inch drives often are repairable, but the cost to do so exceeds the cost of replacing the drive.