I used to boot zfs on whole disk(s) from a USB stick with a GPT partition scheme and gptzfsboot on it since FreeBSD version 9 on many testing machines and even production servers without any problems. With the release 12 this feature is broken.
I found the reason in the source code: In the file /head/stand/i386/libi386/biosdisk.c a regression was introduced in the revisions base r37271, base r337317. Its about detecting the correct number of sectors/blocks for a block device. The change was introduced because a certain device (Mediasonic HD3-U2B PATA to USB enclosure) was not working. I tested various SASTA disks, even a 10TB disk, and USB Sticks, all detecting the wrong number of sectors instead. Reverting the change leaded to the correct detection of the number of sectors and therefore I was able to boot again. I tested also revision base r342785 of the file /head/stand/i386/libi386/biosdisk.c: The detection of the number of sectors is implemented the right way again and has improved of course.
I propose the following patch to fix the regression for FreeBSD 12
--- stand/i386/libi386/biosdisk.c (revision 343578)
+++ stand/i386/libi386/biosdisk.c (working copy)
@@ -270,8 +270,8 @@
total = (uint64_t)params.cylinders *
params.heads * params.sectors_per_track;
- if (total > 0 && bd->bd_sectors > total)
- bd->bd_sectors = total;
+ if (bd->bd_sectors < total)
+ bd->bd_sectors = total;
ret = 1;