Ex Bibliotheca

The life and times of Zack Weinberg.

Friday, 16 May 2003

# 7:10 AM

the backup blues

About three weeks ago I bought a whole bunch of gadgets on E-bay. When hooked together properly, they were supposed to become a tape backup system for my computer. I have had far too much fun getting all of these (see the previous discussion of package delivery annoyances for one such incident) but now I have everything I need: a working SCSI controller, a tape drive, the cable to connect them, and tapes to feed the drive.

So today I plugged everything together and turned it on. The tape drive made an odd flapping noise and started flashing half of its many blinkenlights. This, according to the manual, indicates a hardware fault. The only documented way to find out what kind of hardware fault is to use a special utility to read detailed error reports over the SCSI bus. But when I actually did that I get "BugCheck Error: A209" followed by several lines of hex dump. Gee, really helpful. No, code A209 does not appear in the manual.

Having suspicions about the flapping noise, I took the drive apart, and they were confirmed. For you to understand what was wrong with it, I have to explain the guts of a DLT drive in a bit of detail. DLT is a good design, except for one detail that leaves me wondering what the hell they were thinking? That detail is, the tape cartridges have only one spool apiece. The other spool is permanently fixed inside the drive. The designers did this for a noble reason: it saves storage space. The cartridges would have to be twice as big if they had two spools.

However, the implications of that decision are nasty. Since the cartridge only has one spool, the tape has a free end. Somehow the tape must get threaded through the drive and onto the other spool. The mechanism that does this is extremely clever. The free end of the tape has a hole in it. The takeup spool has a springy metal strip attached to it, with a hook on the end. This strip is threaded through the tape path. When you insert a cartridge, the hook is supposed to grab the hole in the end of the tape and and pull the tape through the drive, onto the takeup spool.

It gets more complicated. The hook is held in the right place by another hook, which catches a hole in the end of the strip. That hook is on a plastic swing arm. I can't see all the little pieces because part of the case is in the way, but I think the idea is the tape cartridge pushes the swing arm back, freeing the strip from its hook, just as the hook on the strip catches the end of the tape. In my drive, the strip had come unhooked from the arm without any tape involved. When I turned on the drive, it tested the motor on the takeup spool; since there was nothing stopping it, the strip got dragged through the drive and wound round the spool. The flapping noise was the end of the strip banging on the inside of the case.

The drive was designed to make this fairly easy to fix. You have to take off one cover panel, which is held down only by snap-on hooks (notice a trend?) Then you unwind the strip from the spool and feed it back through the tape path. Getting the strip onto its hook is a little fiddly but not impossible. I did all this...and it still doesn't work. Now what happens is, it powers up just fine, but when I load a tape it scans the tape back and forth for a long time, the "use cleaning tape" light comes on, and when I try to write files to the tape I get I/O errors.

A cleaning tape, of course, is the one thing I didn't buy.