[Windows Phone 8 Development and Hacking] Dude360:
Hope your all having a great Xmas :)

Having a root issue with a 520. Bootlaoder unlocked fine and Mass Storage seems to work (MainOS appears as a drive) but root fails.

Heres the log:

25/12/2015 18:00:41.626: Starting Windows Phone Internals version 1.1.5835.1923
25/12/2015 18:00:41.891: Found device on interface: 0fd3b15c-d457-45d8-a779-c2b2c9f9d0fd
25/12/2015 18:00:41.891: Device path: \\?\usb#vid_0421&pid_0661&mi_02#6&1540b622&0&0002#{0fd3b15c-d457-45d8-a779-c2b2c9f9d0fd}
25/12/2015 18:00:41.906: Found device on interface: 7eaff726-34cc-4204-b09d-f95471b873cf
25/12/2015 18:00:41.906: Device path: \\?\usb#vid_0421&pid_0661&mi_03#6&1540b622&0&0003#{7eaff726-34cc-4204-b09d-f95471b873cf}
25/12/2015 18:00:42.31: Found Lumia. Mode: Normal.
25/12/2015 18:00:42.62: Found device on interface: 53f56307-b6bf-11d0-94f2-00a0c91efb8b
25/12/2015 18:00:42.62: Device path: \\?\ide#diskwdc_wd1002faex-00z3a0___________________05.01d05#5&f437ab5&0&0.0.0#{53f56307-b6bf-11d0-94f2-00a0c91efb8b}
25/12/2015 18:00:42.78: Found device on interface: 86e0d1e0-8089-11d0-9ce4-08003e301f73
25/12/2015 18:00:42.78: Device path: \\?\acpi#pnp0501#1#{86e0d1e0-8089-11d0-9ce4-08003e301f73}
25/12/2015 18:00:43.388: Operator: 000-GB
25/12/2015 18:00:43.404: Manufacturer Model Name: RM-914_eu_euro1_327
25/12/2015 18:00:43.404: Product Code: 059S3T5
25/12/2015 18:00:43.420: Firmware: 3058.50000.1425.0006
25/12/2015 18:00:43.420: IMEI: XXXXX
25/12/2015 18:00:43.451: Public ID: XXXXX
25/12/2015 18:00:43.466: Bluetooth MAC: XXXXX
25/12/2015 18:00:43.482: WLAN MAC: XXXXX
25/12/2015 18:00:43.529: Bootloader Security: Enabled
25/12/2015 18:00:43.544: Simlock: Unlocked
25/12/2015 18:00:49.320: First rebooting phone to Flash mode (then attempt Mass Storage mode
25/12/2015 18:00:49.352: First rebooting phone to Flash mode...
25/12/2015 18:01:05.662: Phone disconnected.
25/12/2015 18:01:12.524: Found device on interface: 0fd3b15c-d457-45d8-a779-c2b2c9f9d0fd
25/12/2015 18:01:12.524: Device path: \\?\USB#VID_0421&PID_066E#5&1dda42a3&0&1#{0fd3b15c-d457-45d8-a779-c2b2c9f9d0fd}
25/12/2015 18:01:12.548: Found Lumia. Mode: Flash.
25/12/2015 18:01:12.571: Effective Bootloader Security Status: False
25/12/2015 18:01:13.15: Rebooting phone to Mass Storage mode
25/12/2015 18:01:13.16: And now rebooting phone to Mass Storage mode...
25/12/2015 18:01:13.17: Reading status from Flash interface was aborted.
25/12/2015 18:01:13.146: Phone disconnected.
25/12/2015 18:01:15.501: Found device on interface: 53f56307-b6bf-11d0-94f2-00a0c91efb8b
25/12/2015 18:01:15.501: Device path: \\?\USBSTOR#Disk&Ven_Qualcomm&Prod_MMC_Storage&Rev_1.00#7&5313ab4&0&1234567890ABCDEF&0#{53f56307-b6bf-11d0-94f2-00a0c91efb8b}
25/12/2015 18:01:16.849: Found device on interface: 53f56307-b6bf-11d0-94f2-00a0c91efb8b
25/12/2015 18:01:16.850: Device path: \\.\F:
25/12/2015 18:01:22.960: Found Lumia. Mode: Mass storage.
25/12/2015 18:01:22.989: Applying Root Unlock to EFIESP...
25/12/2015 18:01:23.21: Attempt patch: RootAccess-EFIESP
25/12/2015 18:01:23.201: Apply: 8.10.12393.890
25/12/2015 18:01:23.206: Applying Root Unlock to MainOS...
25/12/2015 18:01:23.206: Attempt patch: RootAccess-MainOS
25/12/2015 18:01:24.210: Apply: 8.10.12393.890
25/12/2015 18:01:24.301: Attempted to perform an unauthorized operation.
25/12/2015 18:01:24.305: at System.Security.AccessControl.Win32.SetSecurityInfo(ResourceType type, String name, SafeHandle handle, SecurityInfos securityInformation, SecurityIdentifier owner, SecurityIdentifier group, GenericAcl sacl, GenericAcl dacl)
at System.Security.AccessControl.NativeObjectSecurity.Persist(String name, SafeHandle handle, AccessControlSections includeSections, Object exceptionContext)
at System.Security.AccessControl.NativeObjectSecurity.Persist(String name, AccessControlSections includeSections, Object exceptionContext)
at System.Security.AccessControl.FileSystemSecurity.Persist(String fullPath)
at ..( )
at WPinternals.PatchEngine.(String )
25/12/2015 18:01:24.307: Failed to apply Root Unlock on MainOS!
25/12/2015 18:01:33.63: Applying Root Unlock to EFIESP...
25/12/2015 18:01:33.63: Attempt patch: RootAccess-EFIESP
25/12/2015 18:01:33.112: Apply: 8.10.12393.890
25/12/2015 18:01:33.112: Applying Root Unlock to MainOS...
25/12/2015 18:01:33.112: Attempt patch: RootAccess-MainOS
25/12/2015 18:01:33.426: Apply: 8.10.12393.890
25/12/2015 18:01:33.431: Attempted to perform an unauthorized operation.
25/12/2015 18:01:33.432: at System.Security.AccessControl.Win32.SetSecurityInfo(ResourceType type, String name, SafeHandle handle, SecurityInfos securityInformation, SecurityIdentifier owner, SecurityIdentifier group, GenericAcl sacl, GenericAcl dacl)
at System.Security.AccessControl.NativeObjectSecurity.Persist(String name, SafeHandle handle, AccessControlSections includeSections, Object exceptionContext)
at System.Security.AccessControl.NativeObjectSecurity.Persist(String name, AccessControlSections includeSections, Object exceptionContext)
at System.Security.AccessControl.FileSystemSecurity.Persist(String fullPath)
at ..( )
at WPinternals.PatchEngine.(String )
25/12/2015 18:01:33.432: Failed to apply Root Unlock on MainOS!

Just for the record, my process if it helps

Reflashed a stock ffu (RM914_3058.50000.1425.0006_RETAIL_eu_euro1_327_06_452364_prd_signed) before starting, ran through the usual setup nonsense (didnt sign in) and rebooted a couple of times to make sure the stock ffu was working fine.

Used the same ffu in WPI, Engineering-SBL3-Lumia-520 from a previous post and selected the folder full of hex files.

Ran the bootloader unlock and all seemed well, Mass storage mode worked. Made a backup from within WPI

Enjoy the rest of your holidays people :)