In "download", where does the "down" direction come from?
Initially, "download" and "upload" were used in aviation, especially by the US military. "Download" meant to remove items such as weapons from the aircraft, while "upload" meant to load items onto the aircraft.
For example, the August 1963 Aerospace Maintenance Safety (a publication of the US Air Force) says at page 18:
Failure to follow written procedures and download the missiles...
(meaning failure to remove the missiles from the aircraft)
There are earlier examples of "download", "downloading" and "uploading" in the January 1961 Aerospace Accident and Maintenance Review, also a USAF publication.
And still earlier in the October 1959 Aircraft Accident and Maintenance Review, USAF, at page 27:
During downloading of armament for a routine check, it was discovered that all the missiles aboard the F-102 had their internal power source activated.
(There was also an even earlier meaning relating to the direction of load on an aircraft component, such as on the tail of the aircraft. See "download on tail" in the April 1957 NACA Technical Note 3961 and ""download applied to the horizontal tail surface" in the 1952 US Code of Federal Regulations ).
Then, within the US Air Force, the concept was extended to computers.
The July 1968 IMPLEMENTATION OF THE USAF STANDARD BASE SUPPLY SYSTEM: A QUANTITATIVE STUDY says:
ADC provided a three-man team, which visited the bases some 30 days prior to conversion and conducted a full-scale download of the 305 and upload of the 1050, requiring 10 to 15 days.
...
- Download records from the previous computer
- Upload records on the 1050 computer
OED noticed the word for the first time in 1976, it seems.
1976 was very early in the development of computers. FTP (File Transfer Protocol) was designed 1971 and TCP/ IP followed in 1973. These two protocols made any transfer of data possible for the first time.
Servers or internet wasn't something anyone could imagine yet. Instead scientist were still hammering out the basic how of a data transfer.
In these years, several conceptual models for the transfer of data were developed. Most used and known nowadays is the OSI model with 7 layers - and these layers have an up / down direction.
Now, if I want to get data from another computer, I technically send the other computer a request, which then converts said data "down" to the physical layer, until all is left is a physical signal. This signal is then send to my computer, and I convert it again (this time "upwards").
This also is a likely explanation why "upload" appeared years later, in 1980 according to OED. Upload was coined as the simple opposite of download.
Though, while this is what I first thought of, it might be that there is also a hierarchy meaning in play as well.