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Booting from a USB device on a SPARC system should work, in theory.

The Sun USB-FAQ states:

25. Can I boot from a USB disk or USB CD/DVD drive?
Generally speaking, booting from USB disk, CD/DVD drive is supported on x86 platforms with S10 update 1 and later if the BIOS has USB 2.0 support. Sparc systems with USB 2.0 support can boot from USB disks if the OBP(Open Boot Prom) is upgraded to version 4.27 or later. Before you can boot Solaris from USB disk, you have to install Solaris on the disk first. Installing on USB memory sticks is not recommended as the number of writes to these sticks is limited.
Problems observed on some hardware platforms:
-Installation from USB CD/DVD drive starts, but then fails with "not finding media" error. This is because the CD/DVD drive is not mounted under the /cdrom directory as the installation process expects. Possible workaround is to re-plug in the CD/DVD drive, and make sure the device is in the device tree and the /dev/dsk/ link is correctly created, and mount it under /cdrom manually, then continue installation by /sbin/install-solaris.
- Installation to USB hard disk seems slow and then hangs somewhere. Check dmesg output if there are repeatedly "timeout" and "tran_err". Such error messages would mean you don't have enough physical memory. The workaround is to install more than 512M physical RAMs or to use S10U2 or later. These releases contain the ehci fix for USB 2.0 disk support. There is also patch available, refer to the General section. Furthermore, latest OpenSolaris releases contain similar fixes for USB 1.x disk.
- Booting from USB hard disk panics on "cannot mount root path". Check if the disk is connected behind a USB hub. Don't use a hub between when booting from USB disk. And check if the bootpath in /boot/solaris/bootenv.rc matches the USB disk drive path. If not, modify bootenv.rc, boot into failsafe and make sure boot_archive on the USB disk drive is updated.

Source: http://www.sun.com/io_technologies/usb/USB-Faq.html

From the same document, question 27 is interesting:

27. When I connect my device to the USB ports on the front panel of my SPARC machine sometimes, it does not work. But the same device works when connected to the USB ports on the back panel.
This is most likely caused by some hardware incompatibility issue. To work around the problem, either connect the device to the front port via a USB hub or connect the device to the back port of the SPARC machine.