Cassette Storage / Floppy Drives
I keep seeing statements that the audio cassette was used early on as a slow mass storage device, but that it had a couple drawbacks used in this way. First it was slow, and second, it tended to stretch the tape and was subject to constant alignment problems. All true of course. Thus, viable business applications had to wait until the floppy disk became available. However, in my experience this was simply not true.
Cassette I/O was simply not used as a mass storage device in a batch processing mode. It was a convenient substitute for paper tape I/O which was awkward, cumbersome and slow. The only common use for cassette I/O was to load Basic (or whatever) into the computer, write a program and then, when through running the program written, dump it back onto a fresh tape to preserve it. Since almost all early Basic languages were interpreters (not compilers), the resultant dump of the program needed only to be read into the computer again.
The point is that the cassette was not used in a batch processing mode to store intermediate results for use with subsequent results of further processing. It was just a place to store the programs one wrote, just like paper tape. Many, if not most cassette interfaces didn't even provide for controlling the motors simple on/off functions, let alone start/stop in either direction.
If you accept the above as simple fact, then it is clear that no IMSAI, ALTAIR, SOL-20, or other personal computer did an effective business application before the availability of floppy disk systems and Operating System. And in spite of the price lists and advertising, there were darn few floppy disk systems available prior to the second half of 1976. I saw a MITS dual floppy system at a CACHE meeting in the spring of 1976, but it didn't have any operating system except that which the owner had written. We were selling eight inch floppy systems with controllers by Digital Microsystems and Gary Kildall's CP/M and C-Basic with IMSAI computers by late summer or early fall.
A little later Don Tarbell came out with a controller that was simple and inexpensive, and was sold with CP/M and C-Basic or Tarbell Disk Basic. Since the Tarbell cassette interface and Tarbell cassette basic were very popular, his disk subsystems were immediate successes too.