Booting from a GPT partitioned disk that can have more
than 128 parition entries (e.g. 248 in this case)
fails under 9.1 but worked under 7.4.

This is due to a test that was introduced in sys/boot/common/gpt.c:

...
#define MAXTBLENTS      128
...
        if (bcmp(hdr->hdr_sig, GPT_HDR_SIG, sizeof(hdr->hdr_sig)) != 0 ||
            hdr->hdr_lba_self != hdrlba || hdr->hdr_revision < 0x00010000 ||
            hdr->hdr_entsz < sizeof(struct gpt_ent) ||
            hdr->hdr_entries > MAXTBLENTS || DEV_BSIZE % hdr->hdr_entsz != 0) {
                printf("%s: invalid %s GPT header\n", BOOTPROG, which);
                return (-1);
        }

Fix: 

The check should be removed since a GPT header with more than
128 possible parition entries is not invalid and can legally
be created with gpart.
How-To-Repeat: 
Create a disk using

gpart create -s gpt -n 248 $disk

and add bootcode and system to it. Add some delay into gptinit()
of sys/boot/i386/gptboot/gptboot.c so you can read the error
messages before reset occurs:

invalid primary GPT header
invalid backup GPT header
unable to load GPT