I dual boot win 7 and win xp from 2 separate hard drives on the same computer.  GPT format, UEFI BIOS.
It is done by going into the BIOS boot options menu (f9 on this box) and selecting the SATA drive with the desired OS. 
It is not necessary to select AHCI or MBR, just select the correct HDD. 
A couple of months ago the option to boot from the win xp drive disappeared from the boot options menu.
The only unusual thing I did at the time was to upgrade my Macrium software at a prompt and then restart. 
The win xp drive is still visible in win 7 windows explorer.  All the files are there. Just not seen by the BIOS, and won't boot

Is it possible that the Macrium upgrade made a change to the boot sector so it now fails to see the win xp drive? 

Can't speak definitively on whether a Reflect update could have done that, but did you maybe tinker with the "Rescue environment boot menu option" in the course of the update?  Otherwise is there any chance a BIOS update might have been applied even automatically by a vendor background update management application? And given that this apparently happened months ago, are you sure you'd remember anything else that might have happened around then other than the Reflect update?  The reason I ask about the BIOS update is that in order to boot XP on a system configured to boot in UEFI mode, you'd have to have the Compatibility Support Module enabled (aka CSM or "Legacy Option ROMs", depending on the system), which allows you to boot EITHER legacy BIOS (by specifying a boot device) or UEFI environments -- and I'm wondering if that might have been turned off somehow.

(Minor note: AHCI and MBR are not two different modes of the same thing.  AHCI is a hard drive addressing protocol, whereas MBR is a hard drive partitioning scheme.)

1. tinker with the "Rescue environment boot menu option ...
no, have been using it same way since Macrium was installed
2. any chance a BIOS update might have been applied ...
If so I didn't know it, and have not kept a record of the bios version. There is such an application, it's from HP and it checks for updates, don't think it ran that day. 
3. are you sure you'd remember anything else ...
Not sure, as in bet the farm on it, but I thought about it quite a bit at the time and didn't remember anything that seemed relevant.
There is one thing, when I restarted after the Macrium update, I had the xp drive unplugged.  Wouldn't boot after that, didn't show in the BIOS
4. Last paragraph above ...
I didn't specifically enable the Compatibility Support Module, but it may have been enabled already.  Also didn't turn it off. 
After much wrangling to get the correct drivers for win xp, all I had to do was select the correct HDD, and the OS on it would boot. 

And thanks for the help, hope we can fix this. 

I've seen it happen with my own multi-boot setup, login123, which involves several independently bootable OS drives similar to yours.  Just unplugging and replugging an internal SATA3-connected drive between system start-ups can sometimes "confuse" the machine's drive detection and boot ordering.  It doesn't necessarily require any boot sector or other changes to any drive's contents.  In fact, even if merely upgrading your Reflect application could modify that somehow (highly unlikely) that alone wouldn't stop the drive from showing up in a system setup that included a WinXP installation in its independently bootable options.  In that case, your UEFI "BIOS" listing should include and show all available drives, even including optical media drives that are completely empty.

If one or more of your drives is missing from that EUFI "BIOS" listing with CSM enabled, it's not detecting them all properly and that could be due to either a physical issue (the drive, its connection and/or the port to which it's connected) or an issue with the EUFI "BIOS" itself.  The fact that its contents can be seen by another fully loaded Windows OS doesn't completely rule out that possibility.  I'd suggest unplugging and replugging again (with a replacement cable if you have one) as a relatively simple first step in your attempts to pin it down.  Or try a different connection port if your system has another one available.

Regards, Richard V. ("Arvy")

OK, thanks Arvy.  I'll try that. 
I tried a different sata cable, no luck, but will get another one and try it.
Also will try plugging the xp HDD into a different sata port on the motherboard. 
Also will plug in another HDD and see if it shows up in the boot menu

Fwiw, all the necessary boot files seem to be present in the win xp HDD.

Thanks again.  Will continue checking.  Will dig deeply if not fast. 

That's good 'logical troubleshooting' and usually the most efficient problem solving technique in the long run.  Even if those relatively simple steps don't provide an immediate solution, you will have eliminated that hardware half from any further "digging" that may be required.  Good luck.

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P.S.: If you do need further assistance and if your system's UEFI implementation allows saving a Print Screen image, posting your current boot compatibility settings and boot order listing might permit more specific suggestions to be offered. The examples below are from my own Asus UEFI setup.  Yours will be different, of course.





Regards, Richard V. ("Arvy")