I wanted to back up my Z3 using Hiren's Boot CD. I have done this before, and it worked well.
(I recommend Image For Windows, available on Hiren's, but you should buy a licence. IFW seems to be one of the only programs out there that can actually do a bare metal restore.)
Oddly enough, when I booted the Z from the USB stick, I did not see Image for Windows. In fact, I did not see any programs at all. Mini Windows XP ran fine, but all the programs were missing!
Scrounging around the File Manager turned up nada. It looked like the USB stick was not there, even though the machine had booted from it just fine.
The conflict in my case was an SD card that I had added to the system. It seems Mini XP got confused after boot and replaced the USB stick drive with the SD card. XP was not capable of reading the SD card owing to the large capacity, and essentially got stuck, unable to locate or load any of the Hiren's apps.
Removing the SD card (as well as the uber-small Sandisk Cruzer Fit I had added for extra storage) fixed the issue. On that boot, Mini XP correctly loaded the Hirens HBCD menu application as well as a bunch of other stuff related to Hiren's.
So - if you have problems with Hiren's Mini Windows XP not loading Hiren's apps, try removing all of your other USB storage devices first.
Note that I was able to plug in an external USB hard drive for the actual backup - an essential for my Z, as Hiren's 15.1 doesn't support the network adapters in this machine. So you may be able to plug in other USB drives after booting.
[Update]: Ran into the same issue on another machine - this time a desktop that does not have USB thumb drives or SD cards inserted. I was not able to find a fix for that issue.
[Update 2]: Has the same problem re-occur on my Z3 (SVZ13), even with other USB drives / SD cards removed. Oddly, when I switched the Hiren's stick to the other USB port, it started to work. I don't know if this is consistent behavior or not. On my machine, the USB port closest to the front appeared to be the 'working' one.
Also, with Hiren's on the 'wrong' port, MiniXP could not recognize my Sandisk 64 Gb USB stick. It saw it, but claimed it was only 8 Gb big, and unformatted. With Hiren's booting 'right', it found it without issues.