Just tried it again. I have 70GB available. Got on the hotspot, started the download, unplugged the laptop, waited for it to go to sleep, woke it up - same result. I had to accept the EULAs again and it started to download from about 50mb.

Having attempted (unsuccessfully) to write a resumable HTTP/HTTPS downloader, which is what I suspect nsurlsessiond is using behind the scenes - it's really hard to get it right. Meanwhile I expect to be able to start a BitTorrent download, throw the laptop down the stairs, take out the hard drive and be able to successfully resume it on a different computer because that's a protocol that was actually designed for it.

But Apple has the luxury of controlling both the server and the client. Why isn’t it just a matter of using HTTP range requests?

https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requ...

And if they need to get more clever than that, why not have a BitTorrent-like map of chunk hashes. So you download the update, check the hash of the whole thing and if it fails the hash check, download the chunk hashes which will be say a 256 bit hash for every individual 32 MB chunk of the file. And for any chunk that fails the hash check, use a HTTP range request to download it again.

We can only speculate, but my guess is some combination of “didn’t feel the need to” and “the code responsible for this is nested 6 layers deep between 3 different frameworks”.