Installer fails to update to 11.3 with "root image authentication failed"

[Marginal]

Problem report after rebooting after failed install:

panic(cpu 0 caller 0xffffff80169d9a3a): "root image authentication failed (err = 22)\n"@/System/Volumes/Data/SWE/macOS/BuildRoots/2288acc43c/Library/Caches/com.apple.xbs/Sources/xnu/xnu-7195.101.1/bsd/kern/imageboot.c:1019
Backtrace (CPU 0), Frame : Return Address
0xffffffa058903860 : 0xffffff801648e02d 
0xffffffa0589038b0 : 0xffffff80165d48e3 
0xffffffa0589038f0 : 0xffffff80165c4eda 
0xffffffa058903940 : 0xffffff8016432a2f 
0xffffffa058903960 : 0xffffff801648d84d 
0xffffffa058903a80 : 0xffffff801648db43 
0xffffffa058903af0 : 0xffffff8016c9d68a 
0xffffffa058903b60 : 0xffffff80169d9a3a 
0xffffffa058903e80 : 0xffffff80164bc517 
0xffffffa058903fa0 : 0xffffff801643213e 

Process name corresponding to current thread: kernel_task
Boot args: -no_compat_check root-dmg=file:///BaseSystem/BaseSystem.dmg amfi_get_out_of_my_way=1

Mac OS version:
Not yet set

Kernel version:
Darwin Kernel Version 20.4.0: Fri Mar  5 01:14:14 PST 2021; root:xnu-7195.101.1~3/RELEASE_X86_64
Kernel UUID: BB2FFEBA-7D53-301F-A238-9867CA51276F
KernelCache slide: 0x0000000016200000
KernelCache base:  0xffffff8016400000
Kernel slide:      0x0000000016210000
Kernel text base:  0xffffff8016410000
__HIB  text base: 0xffffff8016300000
System model name: iMac14,2 (Mac-27ADBB7B4CEE8E61)
System shutdown begun: NO
Panic diags file unavailable, panic occurred prior to initialization
Hibernation exit count: 0

System uptime in nanoseconds: 3433488659
Last Sleep:           absolute           base_tsc          base_nano
  Uptime  : 0x00000000cca6e517
  Sleep   : 0x0000000000000000 0x0000000000000000 0x0000000000000000
  Wake    : 0x0000000000000000 0x0000001cb9e81a10 0x0000000000000000
last started kext at 1766985074: |IO!BUSBDFU    8.0.4d18 (addr 0xffffff8018b9d000, size 4096)
...

[Marginal]

Worked on third attempt. Maybe the USB disk wasn't unmounted cleanly on the first two.