Hello,
Sorry for the slow response time, but thanks for the ideas. The Seagate drive is MacOS extended (journaled) format. I don't see an EXT4 option available in Apple Disk Utility: only APFS, MS-DOS, or ExFAT -- all with options of case-sensitive, journaled, or encrypted.
Here is part of the syslog from when drive to initiate the Time Machine backup:
Apr 17 21:56:00 cnid_dbd[22324]: Couldn't open extension maping file /usr/etc/extmap.conf
Apr 17 21:56:00 cnid_dbd[22324]: Couldn't load extension -> type/creator mappings file "/usr/etc/extmap.conf"
Apr 17 21:56:01 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.___var.txt"): not an adouble
sx file
Apr 17 21:56:01 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.__badax_var.txt"): not an adouble
sx file
Apr 17 21:56:01 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.__folder_list.txt"): not an adouble
sx file
Apr 17 21:56:01 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.__myname_var.txt"): not an adouble
sx file
Apr 17 21:56:02 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.___var.txt"): not an adouble
sx file
Apr 17 21:56:02 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.__myrouter_var.txt"): not an adouble
sx file
Apr 17 21:56:02 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.__folder_list.txt"): not an adouble
sx file
Apr 17 21:56:02 afpd[22313]: ad_valid_header_osx("/tmp/mnt/sda2/.__myname_var.txt"): not an adouble
sx file
Apr 17 21:56:02 afpd[22313]: AFP logout by badax
Apr 17 21:56:02 afpd[22313]: AFP statistics: 2.86 KB read, 11.82 KB written
Apr 17 21:56:02 afpd[22313]: done
Apr 17 21:56:15 afpd[22352]: AFP3.3 Login by myrouter
Apr 17 21:56:15 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.___var.txt"): not an adouble
sx file
Apr 17 21:56:15 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myrouter_var.txt"): not an adouble
sx file
Apr 17 21:56:15 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__folder_list.txt"): not an adouble
sx file
Apr 17 21:56:15 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myname_var.txt"): not an adouble
sx file
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.___var.txt"): not an adouble
sx file
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myrouter_var.txt"): not an adouble
sx file
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__folder_list.txt"): not an adouble
sx file
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myname_var.txt"): not an adouble
sx file
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 kernel: thfsplus: walked past end of dir
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.___var.txt"): not an adouble
sx file
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myrouter_var.txt"): not an adouble
sx file
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__folder_list.txt"): not an adouble
sx file
Apr 17 21:56:38 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myname_var.txt"): not an adouble
sx file
Apr 17 21:56:44 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.___var.txt"): not an adouble
sx file
Apr 17 21:56:44 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myrouter_var.txt"): not an adouble
sx file
Apr 17 21:56:44 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__folder_list.txt"): not an adouble
sx file
Apr 17 21:56:44 afpd[22352]: ad_valid_header_osx("/tmp/mnt/sda2/.__myname_var.txt"): not an adouble
sx file
Apr 17 21:56:44 afpd[22352]: AFP logout by myrouter
Apr 17 21:56:44 afpd[22352]: AFP statistics: 4.13 KB read, 9.03 KB written
Apr 17 21:56:44 afpd[22352]: done
Apr 17 21:57:02 WLCEVENTD: Deauth_ind 40:B0:76:A5:EC:E8
Apr 17 21:57:02 WLCEVENTD: Auth 40:B0:76:A5:EC:E8
Thanks...