Since launching last Wednesday, iOS 9 has been downloaded to more than 50 percent of devices, making it the “fastest” software in the company’s history, Apple claims. However, it has been plagued by allegations that some devices were bricked during the update process.

Earlier it was reported that the adoption rate of iOS 9 in the first 24 hours matched that of iOS 8. MacRumours reported that about 12 percent of devices upgraded to iOS 9 within a day of its launch, according to data from Mixpanel, GoSquared and Paddle. Apple has now announced that more than 50 percent of the devices have already been upgraded to the latest OS in the first 4 days. In a press release, Philip Schiller, Apple’s senior vice president of worldwide marketing, said iOS 9 “made an amazing start, at a faster download rate by users than any other software release in Apple’s history.” It also announced that the iPhone 6S and 6S Plus will be available in Apple stores starting September 25th.

One week after the initial emergence, iOS 8 became a platform for adoption. In contrast, iOS 7 ran on more than 70 percent of devices within 20 days of its release. iOS 9 has a much smaller download size than iOS 8, and so more users should be encouraged to upgrade their devices. However, many users have complained that they are unable to pass the “Slide to Upgrade” screen when updating their device. The bug is affecting users who are upgrading directly from iOS 7 to iOS 9.

After downloading the update, users are asked to enter a passcode and then “slide to upgrade”. With the iPhone and iPad freezing, many users are stuck at this stage. Apple has posted a solution to the bug on its support website. This requires users to back up their device via iTunes and perform a factory reset. Apple then recommends a clean install of iOS 9 or roll back to iOS 8.4.1 and restore the previous backup. Although it promises to fix this issue in a forthcoming future update, there is no definite date for it.