I installed from the miniroot on a SD card to a USB stick (after figuring out that the SD isn't visible inside the bsd.rd kernel) and ran it for a few days. After an upgrade or two, an unfortunate power-off and then new attempts to reinstall, it would not see the USB from the obsd boot prompt any more.
After a few clean reinstalls of that USB and even a new USB device I still get stuck at: rebooting... U-Boot 2017.01 (Feb 10 2017 - 10:57:13 +1100) DRAM: 944 MiB RPI 3 Model B (0xa02082) MMC: bcm2835_sdhci: 0 reading uboot.env ** Unable to read "uboot.env" from mmc0:1 ** Using default environment In: serial Out: lcd Err: lcd Net: Net Initialization Skipped No ethernet found. starting USB... USB0: Core Release: 2.80a scanning bus 0 for devices... 4 USB Device(s) found scanning usb for storage devices... 1 Storage Device(s) found scanning usb for ethernet devices... 1 Ethernet Device(s) found Hit any key to stop autoboot: 0 U-Boot> run usb_boot USB device 0: Vendor: SanDisk Rev: 1.00 Prod: Ultra Type: Removable Hard Disk Capacity: 14663.6 MB = 14.3 GB (30031250 x 512) ** Bad device specification usb -bootable ** Scanning usb :1... Found EFI removable media binary efi/boot/bootaa64.efi reading efi/boot/bootaa64.efi 75430 bytes read in 78 ms (944.3 KiB/s) ## Starting EFI application at 01000000 ... Scanning disks on usb... Scanning disks on mmc... MMC Device 1 not found MMC Device 2 not found MMC Device 3 not found Found 5 disks >> OpenBSD/arm64 BOOTAA64 0.2 boot> ls stat(sd0a:/.): Device not configured boot> boot cannot open sd0a:/etc/random.seed: Device not configured booting sd0a:/bsd: open sd0a:/bsd: Device not configured failed(6). will try /bsd boot> cannot open sd0a:/etc/random.seed: Device not configured booting sd0a:/bsd: open sd0a:/bsd: Device not configured failed(6). will try /bsd boot> boot> I can still run the default boot and get it to re-run the installation bsd.rd from the SD card, and the SD card must be inserted or otherwise the rpi3 will not do anything at all at boot for me. I can't remember if the line ** Bad device specification usb -bootable ** used to be there before, or if that somehow is an indication of why the obsd loader can't read the usb filesystem later on or not. Will continue looking for where this comes from but if anyone else knows what buttons I may have pushed or need to push, you're welcome... -- May the most significant bit of your life be positive.