On Twitter, a number of users reported that updates would fail after several minutes, requiring them to start the process over, and many posted screen shots of the error message they received, "Software Update Failed."

The problem was likely caused by overloaded servers that were flooded when too many people tried to download iOS 9 simultaneously, say some tech analysts.

"It's like the Black Friday thing. Some websites get creamed on the traffic on Black Friday," said Bob O'Donnell of Technalysis Research.

Apple did not respond to a request for comment late Thursday.

Ben Bajarin, an analyst with Creative Strategies, said users could wait a few hours or a day – after the hype had died down a bit - and download iOS 9 just fine.

Despite any troubles, significant numbers of iOS users had upgraded; more than 16 percent, according to Mixpanel, a San Francisco, California-based analytics company, as of 4 p.m. PDT (2300 GMT) Thursday.

Still, some analysts call the glitch surprising for Apple; following problems with the iOS 8 upgrade, the company should have been better prepared. IPhone and iPad users who tried to upgrade to iOS 8 encountered one bug after the next, and Apple had to release several further updates, and even then some of the problems were never fixed, say tech analysts.

"It was a little surprising," O'Donnell said. "You would have presumed that they would have done everything in their power to make sure that kind of a thing didn't happen again."

Metering the upgrade, or allowing users to upgrade in waves rather than all at once, would have been a smarter approach, O'Donnell said.

"It's a lot about setting expectations," he said. "Apple didn't do any of that. They just let it go, and boom, people had problems."

(reuters.com)