"sd1i" will only exist on the if there is a FAT or EXT2/EXT3 MBR partition on the device. Don't assume it, check it.
Look at the output of:
sudo fdisk sd1
This will show you what your MBR partition table has within it.
Look at the output of:
sudo disklabel sd1
This will show you what real BSD partitions are on the drive, or, what virtual BSD partitions are on the drive, having been created by the kernel from FAT or EXT2/3 MBR partitions.
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If you have a valid MBR partition, but it is an NTFS filesystem, then you will need a custom kernel to mount it on OpenBSD. NTFS has been considered "experimental, read-only" for many years on OpenBSD.