After a few more attempts at booting my iMac, which failed, then trying to boot into Safe Mode, it eventually loaded my login screen in Safe Mode.
So I was able to run EtreCheck:
EtreCheck version: 2.9.3 (253)
Report generated 2016-02-05 17:43:34
Download EtreCheck from http://etrecheck.com
Runtime 1:22
Performance: Excellent
Click the [Support] links for help with non-Apple products.
Click the [Details] links for more information about that line.
Problem: Computer is restarting
Description:
Computer fails to boot.
Hardware Information: ⓘ
iMac (Retina 5K, 27-inch, Mid 2015)
[Technical Specifications] - [User Guide] - [Warranty & Service]
iMac - model: iMac15,1
1 3.3 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
BANK 0/DIMM1
Empty
BANK 1/DIMM1
Empty
Bluetooth: Good - Handoff/Airdrop2 supported
Wireless: Unknown
Video Information: ⓘ
AMD Radeon R9 M290 - VRAM: 2048 MB
iMac 5120 x 2880
System Software: ⓘ
OS X El Capitan 10.11.3 (15D21) - Time since boot: less than an hour
Disk Information: ⓘ
APPLE HDD ST1000DM003 disk0 : (1 TB) (Rotational)
EFI (disk0s1) <not mounted> : 210 MB
Internal (disk0s2) /Volumes/Internal : 999.86 GB (715.79 GB free)
SanDisk SSD U100 256GB disk1 : (256.06 GB) (Solid State - TRIM: No)
EFI (disk1s1) <not mounted> : 210 MB
Lacie (disk1s2) / : 255.20 GB (209.61 GB free)
Recovery HD (disk1s3) <not mounted> [Recovery]: 650 MB
USB Information: ⓘ
Apple Inc. BRCM20702 Hub
Apple Inc. Bluetooth USB Host Controller
Apple Inc. FaceTime HD Camera (Built-in)
Razer Razer Abyssus
Thunderbolt Information: ⓘ
Apple Inc. thunderbolt_bus
LaCie Rugged mini
Gatekeeper: ⓘ
Anywhere
Kernel Extensions: ⓘ
/Library/Extensions
[not loaded] com.citrix.kext.gusb (1.0) [Support]
System Launch Agents: ⓘ
[loaded] 160 Apple tasks
[running] 76 Apple tasks
System Launch Daemons: ⓘ
[failed] com.apple.watchdogd.plist [Details]
[loaded] 192 Apple tasks
[running] 94 Apple tasks
Launch Agents: ⓘ
[loaded] com.adobe.AAM.Updater-1.0.plist [Support]
[loaded] com.adobe.AdobeCreativeCloud.plist [Support]
[loaded] com.citrix.AuthManager_Mac.plist [Support]
[loaded] com.citrix.ReceiverHelper.plist [Support]
[loaded] com.citrix.ServiceRecords.plist [Support]
[loaded] com.google.keystone.agent.plist [Support]
[loaded] com.teamviewer.teamviewer.plist [Support]
[loaded] com.teamviewer.teamviewer_desktop.plist [Support]
[loaded] org.chromium.chromoting.plist [Support]
Launch Daemons: ⓘ
[loaded] com.adobe.agsservice.plist [Support]
[loaded] com.backblaze.bzserv.plist [Support]
[loaded] com.google.keystone.daemon.plist [Support]
[loaded] com.teamviewer.Helper.plist [Support]
[loaded] com.teamviewer.teamviewer_service.plist [Support]
User Launch Agents: ⓘ
[loaded] com.adobe.AAM.Updater-1.0.plist [Support]
[loaded] com.backblaze.bzbmenu.plist [Support]
[loaded] com.valvesoftware.steamclean.plist [Support]
User Login Items: ⓘ
Steam Application (/Applications/Steam.app)
iTunesHelper Application (/Applications/iTunes.app/Contents/MacOS/iTunesHelper.app)
Dropbox Application (/Applications/Dropbox.app)
Caffeine Application (/Applications/Caffeine.app)
Other Apps: ⓘ
[running] com.apple.xpc.launchd.oneshot.0x10000005.EtreCheck
[running] com.google.Chrome.59552
Internet Plug-ins: ⓘ
CitrixICAClientPlugIn: Version: 12.1.0 - SDK 10.10 [Support]
QuickTime Plugin: Version: 7.7.3
AdobeAAMDetect: Version: 3.0.0.0 - SDK 10.9 [Support]
Default Browser: Version: 601 - SDK 10.11
3rd Party Preference Panes: ⓘ
Backblaze Backup [Support]
remoting_host_prefpane [Support]
Time Machine: ⓘ
Skip System Files: NO
Auto backup: YES
Volumes being backed up:
Lacie: Disk size: 255.20 GB Disk used: 45.60 GB
Destinations:
Noah 2TB [Local]
Total size: 2.00 TB
Total number of backups: 43
Oldest backup: 2013-03-23, 10:27 PM
Last backup: 2015-11-28, 5:07 PM
Size of backup disk: Excellent
Backup size 2.00 TB > (Disk size 255.20 GB X 3)
Top Processes by CPU: ⓘ
4% fontd
1% WindowServer
1% kernel_task
1% Dock
0% SystemUIServer
Top Processes by Memory: ⓘ
958 MB Google Chrome Helper(5)
578 MB kernel_task
467 MB softwareupdated
295 MB WindowServer
221 MB Google Chrome
Virtual Memory Information: ⓘ
1.88 GB Free RAM
6.12 GB Used RAM (3.70 GB Cached)
0 B Swap Used
Diagnostics Information: ⓘ
Feb 5, 2016, 05:29:07 PM Self test - passed
Feb 4, 2016, 12:38:43 PM /Library/Logs/DiagnosticReports/Adobe Lightroom_2016-02-04-123843_[redacted].cpu_resource.diag [Details]
/Applications/Adobe Lightroom/Adobe Lightroom.app/Contents/MacOS/Adobe Lightroom
Feb 3, 2016, 08:39:34 AM /Library/Logs/DiagnosticReports/TeamViewer_Desktop_2016-02-03-083934_[redacted] .crash
/Applications/TeamViewer.app/Contents/Helpers/TeamViewer_Desktop
Feb 2, 2016, 10:41:25 PM /Library/Logs/DiagnosticReports/Adobe Lightroom_2016-02-02-224125_[redacted].cpu_resource.diag [Details]