Additional discoveries
When I removed the defunct internal drive, the external drive disappeared from the drive chain. It turns out that the external drive was unterminated-a label on the back of the enclosure even points that out-so by all rights it should never have worked in the first place. I only got away with it because the SCSI controller was tolerant and the terminating resistors on the internal drive must have damped enough of the signal reflections for it to work normally. Removing the internal drive removed that damping and the controller no longer recognized the external drive.
I got the SCSI controller to recognize the drive again by moving the terminating resistors from the internal drive to the external drive. This was an adventure unto itself as I had to disassemble the enclosure and remove the drive to get access to the bottom. The drive, a Seagate ST41200N is immense and extraordinarily heavy, and the manhandling required to get it out of the enclosure most likely didn't help matters. Getting all those little pins into the holes on the controller board was dicey as the whole pack would often flop over when I applied pressure. I'm lucky I didn't break any of them off.
From the spec sheet:
ST-41200N 94601-12G/M WREN 7 FH UNFORMATTED CAPACITY (MB) ________________1.2G FORMATTED CAPACITY (xx SECTORS) (MB) _____1037 AVERAGE SECTORS PER TRACK ________________71 ACTUATOR TYPE ____________________________VOICE COIL TRACKS ___________________________________28965 CYLINDERS ________________________________1931 HEADS ____________________________________15 DISCS ____________________________________8 MEDIA TYPE _______________________________THIN FILM RECORDING METHOD _________________________ZBR RLL (1,7) TRANSFER RATE (mbytes/sec) _______________1.875-2.875 SPINDLE SPEED (RPM) ______________________3,600 AVERAGE LATENCY (mSEC) ___________________8.33 BUFFER ___________________________________256 Kbyte SCSI-1: Read Look-Ahead, Non_Adaptive, Single-Segmented Buffer SCSI-2: Read Look-Ahead, Adaptive, Multi-Segmented Cache INTERFACE ________________________________SCSI-2 TPI (TRACKS PER INCH) ____________________1600 BPI (BITS PER INCH) ______________________32750 AVERAGE ACCESS (ms) ______________________15 SINGLE TRACK SEEK (ms) ___________________2.5 MAX FULL SEEK (ms) _______________________34 MTBF (power-on hours) ____________________150,000 POWER REQUIREMENTS: +12V START-UP (amps) _4.5 +12V TYPICAL (amps) __1.6 +5V START-UP (amps) __1.1 +5V TYPICAL (amps) ___0.8 TYPICAL (watts) ______24 MAXIMUM (watts) ______60 BUFFERED STEP PULSE RATE (micro sec) _____ WRITE PRECOMP (cyl) ______________________N/A REDUCED WRITE CURRENT (cyl) ______________N/A LANDING ZONE (cyl) _______________________AUTO PARK IBM AT DRIVE TYPE ________________________0 or NONE
It's hard to tell what's going on in this video but the drive is the large, dark-colored box with a bright green LED in the lower left corner. The video makes the drive seem much quieter than it really is in person. It sounds like a turbine when spinning up and makes a disconcerting mechanical *SNAP*...BZZZZZZZT right before it's ready to go and another mechanical *SNAP* as it shuts down. In practice the fast-turning fans in the Amiga's and enclosure's power supplies drowned out the idle spin noise but it was definitely hard to miss when it was doing something.
I'm not going to particularly miss the thing, just the data it held since that was my Amiga's last lifeline. If I had been thinking ahead, I would have replaced the internal drive with a new(er) one and copied everything back to that. Too late now...