Upon further investigation, it seems to be the placement of the grub.cfg file, not of the kernel, that's causing GRUB to flake out. Using debugfs, I found that grub.cfg resides at blocks 748716118 and 748716119. Given a 4KiB block size, that works out to about the 2.79TiB mark on the disk, which is presumably above a GRUB or BIOS 2^32 sector limit. Attempting to do an "ls (hd0,gpt2)/boot/grub" from the "grub rescue>" prompt results in the same "attempt to read or write outside disk" error message noted earlier.
This therefore looks like a GRUB and/or BIOS limitation; however, because larger disks are becoming increasingly common, I believe it's prudent to work around this bug in Ubiquity, at least unless and until a workaround or fix can be added to GRUB.
Upon further investigation, it seems to be the placement of the grub.cfg file, not of the kernel, that's causing GRUB to flake out. Using debugfs, I found that grub.cfg resides at blocks 748716118 and 748716119. Given a 4KiB block size, that works out to about the 2.79TiB mark on the disk, which is presumably above a GRUB or BIOS 2^32 sector limit. Attempting to do an "ls (hd0,gpt2) /boot/grub" from the "grub rescue>" prompt results in the same "attempt to read or write outside disk" error message noted earlier.
This therefore looks like a GRUB and/or BIOS limitation; however, because larger disks are becoming increasingly common, I believe it's prudent to work around this bug in Ubiquity, at least unless and until a workaround or fix can be added to GRUB.