Everything worked fine until one Tuesday morning when I came down to take out the Monday backup tape and replace it with Tuesday's. The backup program was asking for a second tape for Monday's backup. Oh, no! Our files had finally exceeded the capacity of one tape!
This was a bad situation, because we had only a single-tape deck and putting a second tape in after the workday was underway would be useless. So I spent that day looking for files I could delete (there were always plenty) to get it back down to one tape's worth. I must have done well; Tuesday's backup needed just one tape.
Things went fine until I went down to change out the next Monday's backup, and discovered it wanted another tape. This time I did a painstaking analysis of that night's backup logs and determined that there was essentially no more data to back up that night than any other. Must be a bad tape, I thought. So I swapped it for a new one.
Next Monday, same thing -- the new tape had also "run out of space." I considered all sorts of voodoo causes, one of them being that we had changed to daylight-saving time the weekend before the Monday when this all started, but I couldn't get that to make any sense, either. I jumped on to a VAX/VMS discussion group on the Web, and described my situation in detail, including all of the possibilities I had ruled out, but no one had any fresh ideas.
This went on for about a year. We changed back to standard time: no difference. Tried other new tapes but always had the same problem: Monday backups would not fit on one tape, while all others did.
One day, while doing something else, I had a Eureka moment. I went down to the computer room to check, and found what I was looking for. Took care of it, and never had the problem again.
What I hadn't mentioned to the VAX/VMS board, nor to you, was my labeling practice. When we first set up the 8mm backup procedures, I carefully typed the tape IDs on the supplied labels ("Monday," "Tuesday," "Week 1," and so on) to make everything all nice and professional looking. When I retired a tape to replace it with a new one, I would peel the label off the old one and stick it on the new one, to avoid having to type new labels. As time went by, the sticky on the back of the labels would get less sticky during a transfer, and I would supplement it with a little clear cellophane tape to hold the label on securely.
The bottom side of any kind of tape cartridge has several holes, many of them there to mate with pins in the deck to ensure proper alignment. Others are there to tell the deck about specific characteristics of the tape cartridge. One of these holes is there to tell the deck that it is a high capacity tape. A little corner of the cellophane tape that held on my label was covering this hole, making the deck think the tape had only half the capacity it actually had. So it stopped writing to the tape when it reached this amount of data, and dutifully asked for another one. I had innocently transferred this same tape and label piece to each tape as I replaced it, ensuring that I transferred the problem to each new tape. It wasn't much, but it was just enough. When I cut off the little tag of cellophane tape that had been covering the hole, my Monday problem went away.
I returned to the VAX/VMS board to update everyone on my solution. There wasn't much response, but one I found disturbing. "Wow, I'm really surprised you admitted to doing such a dumb thing!" Well, my ego isn't so sensitive that I can't admit to brain freezes, so I was puzzled by his anti-candor attitude. But my question to him, and to you, is: Would you have figured it out sooner?