Can't read GPT header on rpi3

[bob prohaska]

When rebooting an rpi3 running -current as of a couple days ago 
the console sometimes reports

Scanning disk mmc at 7e300000.blk...
Scanning disk usb_mass_storage.lun0...
*** ERROR: Can't read GPT header ***
part_get_info_efi: *** ERROR: Invalid GPT ***
*** ERROR: Can't read GPT header ***
part_get_info_efi: *** ERROR: Invalid Backup GPT ***
*** ERROR: Can't read GPT header ***
part_get_info_efi: *** ERROR: Invalid GPT ***
*** ERROR: Can't read GPT header ***

and gets stuck in the loop. Power-cycling the Pi3 seems
to clear the difficulty. The microSD card is relatively 
new, a Samsung Evo + 128GB. 

It's only happened about three times, so statistics are
somewhat lacking. If anybody's got a suggestion for things
to check please do.

Thanks for reading,

bob prohaska