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root@debian:~# fdisk -l
Disk /dev/sdb: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Disk model: KINGSTON SA400S3
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x01fc6ade
Device Boot Start End Sectors Size Id Type
/dev/sdb1 * 2048 466862079 466860032 222.6G 83 Linux
/dev/sdb2 466864126 468860927 1996802 975M 5 Extended
/dev/sdb5 466864128 468860927 1996800 975M 82 Linux swap / Solaris
Disk /dev/sdc: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Disk model: ST3500410SV
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xdbf0e6e8
Device Boot Start End Sectors Size Id Type
/dev/sdc1 2048 961118207 961116160 458.3G 83 Linux
/dev/sdc2 961120254 976771071 15650818 7.5G 5 Extended
/dev/sdc5 961120256 976771071 15650816 7.5G 82 Linux swap / Solaris
Disk /dev/sda: 232.9 GiB, 250059350016 bytes, 488397168 sectors
Disk model: Samsung SSD 850
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x80f5754e
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 472743935 472741888 225.4G 83 Linux
/dev/sda2 472745982 488396799 15650818 7.5G 5 Extended
/dev/sda5 472745984 488396799 15650816 7.5G 82 Linux swap / Solaris
By the way, all three drives are legacy boot.
UPDATE: After doing a lot of reading, and not necessarily understanding everything I read, it appears that on modern systems the boot flag may not be required. Doesn't answer why two drives have the boot flag and one doesn't but it does explain why the boot flag may not be necessary. It may be that my motherboard, which can handle a mix of UEFI and Legacy drives, doesn't care about the drive not having a boot flag.