Kernel Panic - Why is it happening?

I have suddenly experienced a kernel panic on my Macbook Air mid 2012 today

Running OS X 10.8.5

Can anyone figure why? Suddenly the screen froze and the computer restarted with the message "Your computer restarted because of a problem".

I was browsing the internet on Firefox at the time it happened and had MS Word and Preview open.

The report is here, I could only get this report after logging in as the Administrator Account. My external hard drive used for Time Machine is now not working and can't be repaired by Disk Utility :-(

Thanks in advance for any insights as to why this happened.

Anonymous UUID: 551C960E-D743-FF7F-48D3-DC008E7A30AC

Wed Jul 29 18:10:49 2015

panic(cpu 2 caller 0xffffff800885d117): "complete() while dma active"@/SourceCache/xnu/xnu-2050.48.19/iokit/Kernel/IOMemoryDescriptor.cpp:255 9

Backtrace (CPU 2), Frame : Return Address

0xffffff812c68bb60 : 0xffffff800841d6d6

0xffffff812c68bbd0 : 0xffffff800885d117

0xffffff812c68bc10 : 0xffffff7f88a16a7c

0xffffff812c68bc60 : 0xffffff7f88a15400

0xffffff812c68bd00 : 0xffffff7f88a13a34

0xffffff812c68bd30 : 0xffffff800852767b

0xffffff812c68bdb0 : 0xffffff80085193cd

0xffffff812c68be00 : 0xffffff800850f3a9

0xffffff812c68be50 : 0xffffff800877eaae

0xffffff812c68bef0 : 0xffffff800877ec40

0xffffff812c68bf50 : 0xffffff80087e986a

0xffffff812c68bfb0 : 0xffffff80084cf8a3

Kernel Extensions in backtrace:

com.apple.iokit.IOStorageFamily(1.8)[5BA4CD36-E96D-3A9E-ADFF-A863BBD63BC7]@0xff ffff7f88a09000->0xffffff7f88a2dfff

BSD process name corresponding to current thread: diskmanagementd

Mac OS version:

12F2542

Kernel version:

Darwin Kernel Version 12.6.0: Wed Mar 18 16:23:48 PDT 2015; root:xnu-2050.48.19~1/RELEASE_X86_64

Kernel UUID: 3E1B7058-5CFD-38F3-8B25-4CCD9B29F764

Kernel slide: 0x0000000008200000

Kernel text base: 0xffffff8008400000

System model name: MacBookAir5,2 (Mac-2E6FAB96566FE58C)

System uptime in nanoseconds: 920494802868

last loaded kext at 79594768010: com.apple.driver.AppleUSBCDC 4.1.23 (addr 0xffffff7f88ea0000, size 20480)

last unloaded kext at 201206390767: com.apple.driver.AppleFileSystemDriver 3.0.1 (addr 0xffffff7f8a092000, size 8192)

loaded kexts:

com.symantec.kext.SymAPComm 12.2f1

ng.uds.netusb.controller 1.59

com.seagate.driver.PowSecLeafDriver_10_5 5.2.6

com.symantec.kext.ips 3.5f1

com.symantec.kext.ndcengine 1.0f1

com.symantec.kext.internetSecurity 5.2f1

com.seagate.driver.PowSecDriverCore 5.2.6

com.apple.driver.AudioAUUC 1.60

com.apple.iokit.IOBluetoothSerialManager 4.1.7f6

com.apple.filesystems.autofs 3.0

com.apple.driver.AGPM 100.13.14

com.apple.driver.ApplePlatformEnabler 2.0.7d2

com.apple.driver.X86PlatformShim 1.0.0

com.apple.driver.AppleMikeyHIDDriver 124

com.apple.driver.AppleHDA 2.4.7fc4

com.apple.iokit.BroadcomBluetoothHostControllerUSBTransport 4.1.7f6

com.apple.driver.AppleSMCPDRC 1.0.0

com.apple.driver.AppleSMCLMU 2.0.3d0

com.apple.driver.AppleLPC 1.6.3

com.apple.driver.AppleMikeyDriver 2.4.7fc4

com.apple.driver.AppleSMBusPCI 1.0.11d1

com.apple.driver.AppleUpstreamUserClient 3.5.12

com.apple.iokit.IOUserEthernet 1.0.0d1

com.apple.Dont_Steal_Mac_OS_X 7.0.0

com.apple.driver.ApplePolicyControl 3.4.5

com.apple.driver.AppleBacklight 170.3.5

com.apple.driver.AppleMCCSControl 1.1.11

com.apple.driver.AppleIntelHD4000Graphics 8.1.6

com.apple.driver.AppleIntelFramebufferCapri 8.1.6

com.apple.iokit.IOUSBAttachedSCSI 1.0.4

com.apple.driver.AppleUSBTCButtons 237.1

com.apple.driver.AppleUSBCardReader 3.3.1

com.apple.driver.AppleUSBTCKeyboard 237.1

com.apple.AppleFSCompression.AppleFSCompressionTypeDataless 1.0.0d1

com.apple.AppleFSCompression.AppleFSCompressionTypeZlib 1.0.0d1

com.apple.BootCache 34

com.apple.driver.XsanFilter 404

com.apple.iokit.IOAHCIBlockStorage 2.3.5

com.apple.driver.AppleUSBHub 635.4.0

com.apple.driver.AppleUSBEHCI 621.4.6

com.apple.driver.AirPort.Brcm4331 615.20.17

com.apple.driver.AppleUSBXHCI 635.4.0

com.apple.driver.AppleAHCIPort 2.6.6

com.apple.driver.AppleRTC 1.5

com.apple.driver.AppleSmartBatteryManager 161.0.0

com.apple.driver.AppleACPIButtons 1.8

com.apple.driver.AppleHPET 1.8

com.apple.driver.AppleSMBIOS 1.9

com.apple.driver.AppleACPIEC 1.8

com.apple.driver.AppleAPIC 1.7

com.apple.driver.AppleIntelCPUPowerManagementClient 214.0.0

com.apple.nke.applicationfirewall 4.0.39

com.apple.security.quarantine 2.1

com.apple.driver.AppleIntelCPUPowerManagement 214.0.0

com.apple.driver.AppleBluetoothHIDKeyboard 170.2.4

com.apple.driver.IOBluetoothHIDDriver 4.1.7f6

com.apple.driver.AppleHIDKeyboard 170.2.4

com.apple.iokit.IOSerialFamily 10.0.6

com.apple.kext.triggers 1.0

com.apple.driver.DspFuncLib 2.4.7fc4

com.apple.iokit.IOAudioFamily 1.9.2fc7

com.apple.kext.OSvKernDSPLib 1.12

com.apple.iokit.IOBluetoothHostControllerUSBTransport 4.1.7f6

com.apple.driver.AppleHDAController 2.4.7fc4

com.apple.iokit.IOHDAFamily 2.4.7fc4

com.apple.driver.X86PlatformPlugin 1.0.0

com.apple.driver.IOPlatformPluginFamily 5.4.1d13

com.apple.iokit.IOSurface 86.0.5

com.apple.iokit.IOBluetoothFamily 4.1.7f6

com.apple.driver.AppleSMC 3.1.5d4

com.apple.driver.AppleGraphicsControl 3.4.5

com.apple.driver.AppleBacklightExpert 1.0.4

com.apple.driver.AppleSMBusController 1.0.11d1

com.apple.iokit.IOAcceleratorFamily 74.19

com.apple.iokit.IONDRVSupport 2.3.7

com.apple.iokit.IOGraphicsFamily 2.3.7

com.apple.driver.CoreStorage 296.18.2

com.apple.iokit.IOUSBMassStorageClass 3.5.2

com.apple.driver.AppleThunderboltDPInAdapter 2.5.0

com.apple.driver.AppleThunderboltDPAdapterFamily 2.5.0

com.apple.driver.AppleThunderboltPCIDownAdapter 1.3.2

com.apple.driver.AppleUSBMultitouch 237.3

com.apple.iokit.IOUSBHIDDriver 623.4.0

com.apple.driver.AppleUSBMergeNub 621.4.6

com.apple.driver.AppleUSBComposite 621.4.0

com.apple.iokit.IOFireWireSBP2 4.2.5

com.apple.iokit.IOFireWireFamily 4.5.5

com.apple.iokit.IOSCSIBlockCommandsDevice 3.5.6

com.apple.iokit.IOSCSIArchitectureModelFamily 3.5.6

com.apple.driver.AppleThunderboltNHI 1.9.2

com.apple.iokit.IOThunderboltFamily 2.7.7

com.apple.iokit.IOUSBUserClient 630.4.4

com.apple.iokit.IO80211Family 530.5

com.apple.iokit.IONetworkingFamily 3.0

com.apple.iokit.IOUSBFamily 635.4.0

com.apple.iokit.IOAHCIFamily 2.5.1

com.apple.driver.AppleEFINVRAM 2.0

com.apple.driver.AppleEFIRuntime 2.0

com.apple.iokit.IOHIDFamily 1.8.1

com.apple.iokit.IOSMBusFamily 1.1

com.apple.security.sandbox 220.4

com.apple.kext.AppleMatch 1.0.0d1

com.apple.security.TMSafetyNet 7

com.apple.driver.DiskImages 346

com.apple.iokit.IOStorageFamily 1.8

com.apple.driver.AppleKeyStore 28.21

com.apple.driver.AppleACPIPlatform 1.8

com.apple.iokit.IOPCIFamily 2.8

com.apple.iokit.IOACPIFamily 1.4

com.apple.kec.corecrypto 1.0

Model: MacBookAir5,2, BootROM MBA51.00EF.B02, 2 processors, Intel Core i7, 2 GHz, 8 GB, SMC 2.5f9

Graphics: Intel HD Graphics 4000, Intel HD Graphics 4000, Built-In, 512 MB

Memory Module: BANK 0/DIMM0, 4 GB, DDR3, 1600 MHz, 0x80AD, 0x484D5434353153364D465238412D50422020

Memory Module: BANK 1/DIMM0, 4 GB, DDR3, 1600 MHz, 0x80AD, 0x484D5434353153364D465238412D50422020

AirPort: spairport_wireless_card_type_airport_extreme (0x14E4, 0xE9), Broadcom BCM43xx 1.0 (5.106.98.100.17)

Bluetooth: Version 6.1.7f5 15859, 3 service, 21 devices, 3 incoming serial ports

Network Service: Wi-Fi, AirPort, en0

Serial ATA Device: APPLE SSD SM512E, 500.28 GB

USB Device: USB Receiver, 0x046d (Logitech Inc.), 0xc52b, 0x14100000 / 1

USB Device: hub_device, 0x8087 (Intel Corporation), 0x0024, 0x1d100000 / 2

USB Device: hub_device, 0x0424 (SMSC), 0x2513, 0x1d180000 / 3

USB Device: BRCM20702 Hub, 0x0a5c (Broadcom Corp.), 0x4500, 0x1d181000 / 6

USB Device: Bluetooth USB Host Controller, apple_vendor_id, 0x821f, 0x1d181300 / 9

USB Device: Internal Memory Card Reader, apple_vendor_id, 0x8404, 0x1d183000 / 5

USB Device: Apple Internal Keyboard / Trackpad, apple_vendor_id, 0x024c, 0x1d182000 / 4

USB Device: hub_device, 0x8087 (Intel Corporation), 0x0024, 0x1a100000 / 2

USB Device: FaceTime HD Camera (Built-in), apple_vendor_id, 0x8510, 0x1a110000 / 3