Can powering on/off a 1541 damage a disk left inside?

Question

The User's Guide for the Commodore 1541 disk drive includes a vague warning, on page 7, to "remember to always remove the diskette before the drive is turned off or on". Some third-party books and magazines issue the same warning even more forcefully—for example, Jim Strasma, writing in the June 1984 issue of RUN, goes so far as to claim that "if you turn the drive on or off with a disk in place and the door latched, you will probably damage the data on the disk".

Now, I had a Commodore 1541 and I must have turned the drive on or off with a disk inside thousands of times. This never resulted in any obvious damage to the physical disk or to its digital contents. Was I just lucky, or is there really some mechanism by which powering up or down the drive might damage a disk left inside? If so, what exactly is this mechanism, and under what sort of conditions might it be triggered?

Note that I'm not talking about turning off the drive while it has a file open for writing; obviously that can result in things like splat files and corrupted block allocation maps. I'm asking whether it can be dangerous to shut off the drive when no disk activity is ongoing.

Answer 1

According to the 1541 service manual, the device contains power-up and power-down write protection. On power-up, the CPU controls the power to read/write electronics so they are not turned on before CPU is properly running. On power-down, the power to read/write electronics turns off before CPU loses power.

That said, of course it is possible that there are still mechanisms that may damage data, in which case it is best to not to leave disk inside if you want to be sure.

Answer 2

I don't know about the 1541 in particular, but I have personally lost data by powering off a computer with a diskette in the drive. One problem that could occur, independently of whether you remembered to save your work or, whether the OS finished writing out buffered data, was that somewhere in between the power supply and the read/write head, the power-off event could create a "glitch" that would ruin the formatting of the track on which the head was resting.

Answer 3

In the thousands of times I turned my 1541 off and on I always ejected (at least partially) the disk in the drive, because I also heard (probably read somewhere) that leaving it in place could damage the disk. The reasoning I (rarely) heard for this precautionary measure was that when you turn the drive on, nobody can guarantee that no accidental electric pulse (from turning on the whole drive/electronics) could reach the r/w head and affect the magnetic surface on the disk.

Pulling out the disk just halfway always seemed enough for me, as opening the latch also moves the r/w head vertically away from the disk, and - as we know - it has to be very close to read/write data.

Good question and very interesting that you didn't experience any data loss. I couldn't tell because I got into the habit to pull the disk out always. I even had the one or other corrupted file/disk nevertheless, but that's a different story.

Answer 4

I have just discovered that Jim Strasma, the very same author whose 1984 remark prompted my question, provided a more detailed explanation of it in a 1987 RUN article. That article was the October 1987 edition of his "Commodore Clinic" Q&A column. Ben Johnson of Adjuntas, Puerto Rico asks,

I regularly turn my 1541 disk drive off and on with the disk still in the drive. In my view, tile equipment should have been designed to operate this way. Yet both you and my Emerald FSD-1 manual caution against this practice with near-religious fervor. I wish someone would clarify this point once and for all.

Strasma replies,

I agree that the 1541 should be designed to work properly when Switched off and on with a disk in place. Commodore knows how, having designed both its 8050 and 8250 drives properly in this regard. In all early 1541s, however, a serious problem can arise whenever power is lost with a disk in place.

When you cut off the power to your drive, the voltage that's used for save operations lingers longer than the voltage that's used to prevent a save operation from being executed. Thus, there is a small but real chance that erroneous data will be saved to your disk any time you switch off the power with the disk in place or if there's a power failure.

The simplest cure is to release the lever that loads the drive head so that the head is retracted front the disk. Then you won't need to actually remove the disk.

I don't consider Strasma to be an unimpeachably authoritative source on computing matters generally, but when it comes to hardware he seems to know his stuff (and IIRC he authored the revised edition of Commodore's official 1541 User's Guide). So in this case I'm willing to believe that he's got it right.

Strasma's explanation seems to jibe with Justme's answer and provides some additional details that clarify exactly where the problem lies. Note also that he says that the problem affects "early" 1541s, which implies that it was fixed in later iterations of the equipment. This might explain why many heavy users of the 1541 (like me) never encountered the problem, despite always leaving disks in the drive.

---

1. Strasma, Jim. Commodore Clinic. RUN 46:106–108, October 1987.