Hi Alex,
Disk first aid reported no errors at all with the primary drive, annoyingly.
The last section of the System Log for a single boot cycle reads as follows. I don't know how to interpret it, and would be grateful for enlightenment!
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Nov 13 15:54:12 localhost com.apple.xpc.launchd[1]: Early boot complete. Continuing system boot.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.cmio.AVCAssistant): ThrottleInterval set to zero. You're not that important. Ignoring.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.diskmanagementd): ThrottleInterval set to zero. You're not that important. Ignoring.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.diskmanagementd): The TimeOut key is no longer respected. It never did anything anyway.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.xpc.launchd.domain.system): com.apple.CoreRAID (lint): The HideUntilCheckIn property is an architectural performance issue. Please transition away from it.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.syslogd): The HopefullyExitsLast key is no longer respected. Please remove it.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.syslogd): Please switch away from OnDemand to KeepAlive.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.opendirectoryd): Unknown key for Boolean: BeginTransactionAtShutdown
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.xpc.launchd.domain.system): com.apple.opendirectoryd (lint): The HideUntilCheckIn property is an architectural performance issue. Please transition away from it.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.opendirectoryd): This service is defined to be constantly running and is inherently inefficient.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.mobile.keybagd): This key does not do anything: OnDemand
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (org.cups.cupsd): Service has increased maximum shutdown time to 65 seconds. This will create a poor experience for the user.
Nov 13 15:54:12 localhost com.apple.xpc.launchd[1] (com.apple.xpc.launchd.domain.system): Could not import service from caller: path = /System/Library/LaunchDaemons/com.apple.applefileutil.plist, caller = launchd.1, error = 152: Service cannot load in current boot environment
Nov 13 15:54:13 localhost syslogd[57]: ASL Sender Statistics
Nov 13 15:54:13 localhost com.apple.xpc.launchd[1] (com.apple.xpc.launchd.domain.system): Service "com.apple.ManagedClient.startup" tried to register for endpoint "com.apple.ManagedClient.agent" already registered by owner: com.apple.ManagedClient
Nov 13 15:54:13 Als-Mac-Pro xpcproxy[158]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:13 Als-Mac-Pro com.apple.xpc.launchd[1] (com.apple.xpc.launchd.domain.user.202): com.apple.pluginkit.pkd (lint): Service sets EnableTransactions=false and EnablePressuredExit=true, which makes no sense. Enabling Transactions.
Nov 13 15:54:13 Als-Mac-Pro xpcproxy[161]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:13 Als-Mac-Pro xpcproxy[96]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:13 Als-Mac-Pro xpcproxy[111]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:13 Als-Mac-Pro xpcproxy[117]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:13 Als-Mac-Pro logd[74]: _logd_remote_start_service: listener activated for com.apple.logd.remote-daemon
Nov 13 15:54:13 Als-Mac-Pro launchd[1]: BUG in libdispatch client: mach_recv, monitored resource vanished before the source cancel handler was invoked { 0x7f9007f10f90[source], ident: 171523 / 0x29e03, handler: 0x104758444 }
Nov 13 15:54:13 Als-Mac-Pro com.apple.xpc.launchd[1] (com.apple.CoreRAID[64]): Endpoint has been activated through legacy launch(3) APIs. Please switch to XPC or bootstrap_check_in(): com.apple.CoreRAID
Nov 13 15:54:13 Als-Mac-Pro xpcproxy[187]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:13 Als-Mac-Pro syspolicyd[188]: objc[188]: Class SPExecutionHistoryItem is implemented in both /System/Library/PrivateFrameworks/SystemPolicy.framework/Versions/A/SystemPolicy (0x7fffa12b7b80) and /usr/libexec/syspolicyd (0x10de089f8). One of the two will be used. Which one is undefined.
Nov 13 15:54:13 Als-Mac-Pro Installer Progress[84]: Progress UI App Starting
Nov 13 15:54:14 Als-Mac-Pro xpcproxy[82]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:15 Als-Mac-Pro xpcproxy[195]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:19 Als-Mac-Pro mds[81]: objc[81]: Class MDSObjectToken is implemented in both /System/Library/PrivateFrameworks/SpotlightServerKit.framework/Versions/A/SpotlightServerKit (0x7fffa0f6d5f0) and /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/Metadata.framework/Versions/A/Support/mds (0x1060af190). One of the two will be used. Which one is undefined.
Nov 13 15:54:19 Als-Mac-Pro mds[81]: objc[81]: Class MDSToken is implemented in both /System/Library/PrivateFrameworks/SpotlightServerKit.framework/Versions/A/SpotlightServerKit (0x7fffa0f6d5a0) and /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/Metadata.framework/Versions/A/Support/mds (0x1060af6e0). One of the two will be used. Which one is undefined.
Nov 13 15:54:19 Als-Mac-Pro mds[81]: objc[81]: Class MDSReadCopyUpdate is implemented in both /System/Library/PrivateFrameworks/SpotlightServerKit.framework/Versions/A/SpotlightServerKit (0x7fffa0f6d640) and /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/Metadata.framework/Versions/A/Support/mds (0x1060b0270). One of the two will be used. Which one is undefined.
Nov 13 15:54:19 Als-Mac-Pro mds[81]: objc[81]: Class MDSInternalToken is implemented in both /System/Library/PrivateFrameworks/SpotlightServerKit.framework/Versions/A/SpotlightServerKit (0x7fffa0f6d550) and /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/Metadata.framework/Versions/A/Support/mds (0x1060b1350). One of the two will be used. Which one is undefined.
Nov 13 15:54:20 Als-Mac-Pro airportd[198]: DEPRECATED USE in libdispatch client: dispatch source activated with no event handler set; set a breakpoint on _dispatch_bug_deprecated to debug
Nov 13 15:54:22 Als-Mac-Pro WindowServer[190]: DEPRECATED USE in libdispatch client: dispatch source activated with no event handler set; set a breakpoint on _dispatch_bug_deprecated to debug
Nov 13 15:54:22 Als-Mac-Pro com.apple.xpc.launchd[1] (com.apple.xpc.launchd.domain.user.205): com.apple.pluginkit.pkd (lint): Service sets EnableTransactions=false and EnablePressuredExit=true, which makes no sense. Enabling Transactions.
Nov 13 15:54:23 g183-8 xpcproxy[206]: libcoreservices: _dirhelper_userdir: 529: bootstrap_look_up returned (ipc/send) invalid destination port
Nov 13 15:54:23 g183-8 nsurlsessiond[166]: DEPRECATED USE in libdispatch client: Setting timer interval to 0 requests a 1ns timer, did you mean FOREVER (a one-shot timer)?; set a breakpoint on _dispatch_bug_deprecated to debug