"Beach balling" and freeze

I recently upgraded to El Capitan 10.11.6 and today I encountered a problem -- I doubled-clicked on my HD icon and the cursor changed to a beachball. I wasn't able to recover, so I held the power button on my iMac until the system restarted.

I ran EtreCheck and here is the report. Any idea as to what com.apple.watchdogd.plist (2016-02-27) is, and could this be the problem?:

EtreCheck version: 3.0.1 (304)

Report generated 2016-08-17 17:48:50

Download EtreCheck from https://etrecheck.com

Runtime 1:35

Performance: Excellent

Click the [Support] links for help with non-Apple products.

Click the [Details] links for more information about that line.

Problem: Beachballing

Description:

Computer suddenly displayed the “beachball” and became unresponsive.

Hardware Information: ⓘ

iMac (21.5-inch, Late 2013)

[Technical Specifications] - [User Guide] - [Warranty & Service]

iMac - model: iMac14,1

1 2.7 GHz Intel Core i5 CPU: 4-core

8 GB RAM Upgradeable - [Instructions]

BANK 0/DIMM0

4 GB DDR3 1600 MHz ok

BANK 1/DIMM0

4 GB DDR3 1600 MHz ok

Bluetooth: Good - Handoff/Airdrop2 supported

Wireless: Unknown

Video Information: ⓘ

Intel Iris Pro

iMac 1920 x 1080

System Software: ⓘ

OS X El Capitan 10.11.6 (15G31) - Time since boot: less than an hour

Disk Information: ⓘ

APPLE HDD HTS541010A9E662 disk0 : (1 TB) (Rotational)

EFI (disk0s1) <not mounted> : 210 MB

Recovery HD (disk0s3) <not mounted> [Recovery]: 650 MB

Macintosh HD (disk1) / : 999.03 GB (942.84 GB free)

Core Storage: disk0s2 999.35 GB Online

USB Information: ⓘ

Apple Inc. BRCM20702 Hub

Apple Inc. Bluetooth USB Host Controller

Apple Inc. FaceTime HD Camera (Built-in)

Western Digital My Book 1110 499.41 GB

disk2s1 (disk2s1) <not mounted> : 32 KB

disk2s2 (disk2s2) <not mounted> : 29 KB

disk2s3 (disk2s3) <not mounted> : 29 KB

disk2s4 (disk2s4) <not mounted> : 29 KB

disk2s5 (disk2s5) <not mounted> : 29 KB

disk2s6 (disk2s6) <not mounted> : 262 KB

disk2s7 (disk2s7) <not mounted> : 262 KB

disk2s8 (disk2s8) <not mounted> : 262 KB

My Book (disk2s10) /Volumes/My Book : 499.27 GB (126.35 GB free)

APC Back-UPS ES 500 FW:801.e5.D USB FW:e5

Thunderbolt Information: ⓘ

Apple Inc. thunderbolt_bus

Gatekeeper: ⓘ

Mac App Store and identified developers

System Launch Agents: ⓘ

[not loaded] 6 Apple tasks

[loaded] 161 Apple tasks

[running] 71 Apple tasks

System Launch Daemons: ⓘ

[failed] com.apple.watchdogd.plist (2016-02-27)

[not loaded] 46 Apple tasks

[loaded] 155 Apple tasks

[running] 88 Apple tasks

User Login Items: ⓘ

iTunesHelper Application Hidden (/Applications/iTunes.app/Contents/MacOS/iTunesHelper.app)

Internet Plug-ins: ⓘ

Default Browser: 601 - SDK 10.11 (2016-08-14)

QuickTime Plugin: 7.7.3 (2016-08-14)

3rd Party Preference Panes: ⓘ

None

Time Machine: ⓘ

Skip System Files: NO

Mobile backups: OFF

Auto backup: YES

Volumes being backed up:

Macintosh HD: Disk size: 999.03 GB Disk used: 56.19 GB

Destinations:

My Book [Local]

Total size: 499.27 GB

Total number of backups: 270

Oldest backup: 11/2/11, 11:31 AM

Last backup: 8/15/16, 12:16 PM

Size of backup disk: Adequate

Backup size 499.27 GB > (Disk used 56.19 GB X 3)

Top Processes by CPU: ⓘ

2% fontd

1% WindowServer

0% cloudpaird

Top Processes by Memory: ⓘ

645 MB kernel_task

98 MB iconservicesagent(2)

49 MB Dock

49 MB WindowServer

41 MB com.apple.CommerceKit.TransactionService(2)

Virtual Memory Information: ⓘ

5.41 GB Free RAM

2.58 GB Used RAM (1.69 GB Cached)

0 B Swap Used

Diagnostics Information: ⓘ

Aug 17, 2016, 05:36:46 PM Self test - passed

Aug 17, 2016, 05:35:49 PM /Library/Logs/DiagnosticReports/System Preferences_2016-08-17-173549_[redacted].hang

/Applications/System Preferences.app/Contents/MacOS/System Preferences

Aug 17, 2016, 05:32:12 PM /Library/Logs/DiagnosticReports/Finder_2016-08-17-173212_[redacted].hang

/System/Library/CoreServices/Finder.app/Contents/MacOS/Finder