Q: The iPhone could not be restored. An unknown error occurred (48)

Has the software on the phone been modified in the past? If yes, jailbreaking can be a reason for not being able to restore the phone again.

Did you already check this article?

How to Fix iTunes Error 48 When Restoring iPhone ...

Update and restore error (48)

iphone error 48

https://www.apple.com/support/iphone/repair/

Alternately, you could save a substantial sum of money at a reputable non-affiliated repair shop. Some individuals have stated that such action will cause Apple to refuse any and all future service but I have not seen any Apple documentation that supports that statement.  The following is what I have found:

http://www.apple.com/support/service/

Select “iPhone”

http://support.apple.com/kb/index?page=servicefaq&geo=United_States&product=ipho ne

Select “Learn more about service coverage”

http://support.apple.com/kb/index?page=servicefaq&geo=United_States&product=ipho ne&select=WARRANTY__PRICING

Under Out-of-warranty repair service, select “Read Apple’s Repair Terms and Conditions”

http://www.apple.com/legal/sales-support/terms/repair/

Select “General Terms”

http://www.apple.com/legal/sales-support/terms/repair/

Select “English”

http://www.apple.com/legal/sales-support/terms/repair/generalservice/serviceterm sen/

Scroll down to section 1.10.6

1. 1.10.6  Disclosure of Unauthorized Modifications. During the service ordering process, you must notify Apple of any unauthorized modifications, or any repairs or replacements not performed by Apple or an Apple Authorized Service Provider (“AASP”), that have been made to your product.  Apple will not be responsible for any damage to the product that occurs during the repair process that is a result of any unauthorized modifications or repairs or replacements not performed by Apple or an AASP. If damage results, Apple will seek your authorization for any additional costs for completing service even if the product is covered by warranty or an AppleCare service plan. If you decline authorization, Apple may return your product unrepaired in the damaged condition without any responsibility.

Apple will not service the phone anymore if a 3rd party repair shop has opened the phone to service the device.

...

or (iv) service cannot be performed because the serial number has been altered, defaced or removed or the product has failed due to accident, abuse, liquid spill or submersion, neglect, misuse (including faulty installation, repair, or maintenance by anyone other than Apple or an Apple Authorized Service Provider), unauthorized modification, extreme environment (including extreme temperature or humidity), extreme physical or electrical stress or interference, fluctuation or surges of electrical power, lightning, static electricity, fire, acts of God or other external causes (“Service Exclusions”). Apple will return your product to you without servicing it and may charge you the Diagnostic Fee.

copied from: Repair Terms and Conditions