iOS devices like iPhones and iPods utilize NAND flash memory (just like thumb drive or SSD drives). This memory ages over time as it’s overwritten again and again, whether by iMessages, photos or app downloads.
Bits within a block of data on the storage chips get “stuck”, or become error-prone. These bits are normally corrected by Error Correcting Code on the controller, but this has a limit; the system using the NAND may start acting very strangely afterward, with data and system corruption, or an inaccessible drive.
Failures can also arise from bad solder connections. Dropping the phone or constant vibration over time could loosen solder joints on the NAND chips, and solder joints that were poorly soldered at the factory could cause issues as well.
My (now 3-year-old) iPhone 4 has had occasional “bad behavior,” but this was usually rectifiable with a restore of the device. However, for the last couple of months, there have been constant cell network disconnects, forcing me to restart the device or reseat the SIM to get it back. This always came with an “Activation Failed, Restore Your Device” message.
Yesterday, I attempted to restore it, resulting in an Error 37 or a -1. After about three-dozen restore attempts between two platforms, multiple sync cables, iTunes alternatives, different iOS IPSWs, and other techniques, I threw in the towel. While the iTunes errors were inconsistent, the situation pointed at the firmware not being written properly to the NAND memory.
Unfortunately, in these instances, we are forced to replace devices. The NAND built into the main logic board is unrepairable and the entire board needs to be replaced. Yet with the age of iOS devices when they tend to fail in this manner, a new device is often a better resolution, given the cost.
Nothing is permanent, so back up your data!