iPhone error 27 is an error that usually occurs when the device or computer might have a hardware issue that stops the update or restore from completing.
Some of the solutions to this error include restoring your device by using Device Firmware Upgrade (DFU) mode.
Please see our in-depth walk-through solutions below.
Solution 1. Device Firmware Upgrade (DFU) mode
iPhone error 27 or iTunes error 27 may appear when your iOS is corrupted. It may be fixed by doing a Device Firmware Upgrade. But if an iPhone gets stuck on boot up and encounters the error while in DFU mode, the cause may be a hardware failure. The most common cause of this is a NAND chip (also known as a storage device) failure. To confirm hardware failure, try to restore it with DFU mode multiple times.
If the DFU mode did not work, request a warranty claim. If the device is out of warranty, go to your local cellphone repair shop with a chip level repair. They might be able to repair, replace or reprogram your NAND chip.