> It always chokes on fp->fsisig4. Well, that's what you get from reading 512 bytes and casting the buffer to a 1024 byte struct.
The following diff ought to solve this. Index: msdos/msdosfs_vfsops.c =================================================================== RCS file: /OpenBSD/src/usr.sbin/makefs/msdos/msdosfs_vfsops.c,v retrieving revision 1.13 diff -u -p -r1.13 msdosfs_vfsops.c --- msdos/msdosfs_vfsops.c 6 Oct 2021 00:40:41 -0000 1.13 +++ msdos/msdosfs_vfsops.c 1 Dec 2023 06:52:40 -0000 @@ -278,7 +278,8 @@ msdosfs_mount(struct mkfsvnode *devvp, i DPRINTF(("%s(bread %lu)\n", __func__, (unsigned long)de_bn2kb(pmp, pmp->pm_fsinfo))); if ((error = bread(devvp, de_bn2kb(pmp, pmp->pm_fsinfo), - pmp->pm_BytesPerSec, 0, &bp)) != 0) + roundup(sizeof(struct fsinfo), pmp->pm_BytesPerSec), + 0, &bp)) != 0) goto error_exit; fp = (struct fsinfo *)bp->b_data; if (!memcmp(fp->fsisig1, "RRaA", 4)