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Kind of a start.. I was trained on how to fix these back in the late 70’s. Seems they progressed through various ways to determine the tape position.
Visible leds, ir leds and I think they finally found a more dependable solution using a strip along the edge of the tape loop/channel that changed capacitance, depending on where the tape was.
What most don’t realize, is these were mainly batch machines. When you wanted or needed to run a program an operator would start it and your JCL (Job Control Language) would query the operator to hang a specific tape. If you wrote to tape, then the operator would have to mount another tape for that operation. It likely required a specific disk pack.
When I first leaned this, we did it on punched cards.. finally we got terminals we could write JCL and not have someone punch the cards.
If you used any disk space, you had to tell it cylinder, head and sector and ensure you figured out enough space as it was allocated before the job ran.
Memory was core and was done by hand, each bit having a core with x and y wires running through them. So it was expensive.
8 gB sd card sitting on top of a 64 bit (8 bytes) of core memory… over a million times the storage.
This is a photo of a 32x32 (1024 bits) or 128 bytes of core memory.
Disk storage wasn’t low cost either. One of the machines we had that was discontinued before 1970, before I arrived, held about 5 million 6 bit bytes and I was told the original rental cost was $3200/month back in the mid 60’s. Forgot to mention, these weighted in at about a ton, literally, and required a heavy equipment to move around, not to mention power, controller and cabling. Far cry from a memory stick 
We used a number of the Winchester 3030, as they were coined, dual 30mB drives. These were the lowest cost disk drives available. They also cost about 3 to 4k a month or just under $200k if you purchased them outright.
When we got terminal access, it always told us cpu, disk and memory usage and what it cost.
Boy things have changed and will continue to do so at a much faster rate.
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A walk down memory lane for me. I designed storage and other peripherals for my entire career.
.. and there was this!! https://youtu.be/hY5OmDLJJPU?si=LIFW3NYZokC6JPk8
@jkwilborn Thanks for sharing your memories and expertise with us!
@donkjr That tape drive library is truly amazing!
Before the tape library, in big data centers operators on roller skates would retrieve from storage and then mount tapes when requested from the IBM console. As a programmer you had to request a tape mount to run a program.
When we first started, experts laughed at the notion of installing a huge silo in a data center.
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Ah, so that’s how they used to run DOOM on mainframes. 
The 90s were nuts for storage system designers no doubt. So many technological advances that by the time you got a system out it was obsoleted.
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Nothing changes…
In my almost 50 years in these machines, the hardware becomes obsolete and is disposed of before all of the available hardware items are implemented in software. Software never seems to catch up.
We are now and have been for decades, a data generation.
Honeywell had a mass storage device, don’t remember the exact name. Used cartridges that had nearly 70mm wide tape. The machine took the cartridge, picked by vacuum and opened it internally to read it. Full of greasy go-rounds.
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I had friends working at Honeywell in the mid 70’s and they gave me the back door to the Mainframe so I could run adventure from a VT-100 terminal and 300 Baud modem.
I used to have the original B source code for adventure cave game.
I was at I-17 and Thunderbird.
Used to bring Datel (IBM Selectric) console home and connected at 134.5 baud, I think…
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In the late 80s I was working towards a EE degree and switched to CompSci because I saw hardware getting generic and software being the innovative workhorse. Worked my way though schooling as an electronics tech after years as an auto mechanic. The robotics revolution we’re having has been fun to watch.
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Cool! It was rogue on HPUX in the 80s for me.
To continue the saga. We designed tape disk and printer peripherals @ StorageTek.
The market drive was for higher-density disks. We drove the disk technology too far and started taking down data centers due to crashing heads. The company had to enter chapter 11. The new tape library design brought us out of the chapter.
We swore never to crash a disk again … enter RAID storage and ICEBURG, 100-400 GB for a few million $. [The solid state drive in my $1K desktop is 1TB]
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Even though I was not an employee of Honeywell, I played on their volleyball team.
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