Complex sentence: It could have returned an error back to the program, similar to how it would act if the disk was in the drive but the file not found, however most programs at this time simply exited and thus the user could not fix the problem and continue.
Split into simpler sentences: It could have returned an error back to the program, similar to how it would act if the disk was in the drive but the file not found. However, most programs at this time simply exited and thus the user could not fix the problem and continue.