I have two drives, one SATA and one PATA (EIDE)
I have Windows 10, linux and Widnows XP on the SATA drive.
The PATA drive is data only, but does contain the GRUB loader.

If I set the PATA drive to be the first device to boot in the BIOS,
I get the GRUB menu and I can boot Linux normally.

However, since I still boot Windows more often than Linux, I set the SATA drive to be
the first boot device and I get the Windows 10 boot loader and I can choose
Windows 10 or XP.

When I wanted to boot Linux, I could use a second method - press F9 during the POST and
I would get a menu following the post asking me what drive I wanted to boot from.
Here I could select the PATA drive and I could successfuly boot Grub/Linux as before.

But somewhere an error has crept in, this method of choosing Linux no longer works,
I get
GRUB Loading stage 1.5
error 22

However, It still works if I use the BIOS setup to point to the PATA drive as the first
boot device directly.

This is an older PC, using legacy BIOS and MBR
Appreciate any suggestions.

Not many use the old (legacy) grub any more. Memories fade ...
This is caused by device mapping mismatch - I'd say you had a grub update when logged into Linux. Although I'd expect both to still work.
This only seems to happen in a mixed SATA/(E)IDE environment. I've had situations in multi-boot where only one of the Linux systems would successfully boot without intervention. FWIW I'd simply add a grub boot stanza to chainload the Win10 loader, and always just use the Linux disk. If Win10 complains (say during an update), simply change the load order for that update session.

Thank you for this information. This is a 6 year (approx.) old install of PCLinuxOS.

I've kept it updated through their program, Synaptic, one interesting side effect I found. My desktop state-of-the art when I installed the system in 2010.
Even though KDE has been updated numerous times, the desktop never changed it's basic (circa 2010) look. It wasn't until I created a second user recently that
I actually got to see the new KDE desktop default, applied to the new user. This was pointed out to me when I showed a screen print of my desktop, trying to
'troubleshoot something or other - I got asked why I hadn't kept my system up to date. Of course I had.

If I installed their latest dist. directly, what would I get now? Grub2? Does it work on older hardware? Can I update my existing system, to use it?

No, it looks like they keep it up-to-date (I have never used PCLinuxOS). You have simply tripped over a configuration issue. Stick with what you have.
It looks like they are planning a major update which will include grub2 - and yes it supports old systems, and yes it should fix the issue. Best to wait till the developers roll it out though. Maybe pursue this through their fora - you might get a timetable for the upgrade.

Thanks for the advice. I found this, looks like it's already there (the article is from 9/14) - but you have to manually install it.
http://pclosmag.com/html/Issues/201409/page08.html

After downloading grub2 and issuing the two commands, I can now say the problem is resolved. I can leave Windows 10
as the bios default and use F9 when I want to bring up Linux.

Thanks again