Thank You, all, for your replies.
I'll try to explain it all, & answer your questions in doing so:
I purchased the WD drives a few years ago, as they were connectable by firewire. They were used for ages, with no problems.
The Caldigit was added later, as it was a firewire connection, too, specifically for the huge iTunes collection. It does not start up when the mac is turned on, like the WD ones do. Originally, all 4 were daisy-chained.
This arrangement was fine, until May, this year. Upon start-up one day, the 3 WD drives came up as "My Book", & not as their volume names that I had assigned. They also all reported as empty, no matter what I used to explore them. The Caldigit was fine, as I had not turned it on!! I immediately disconnected it, & didn't plug it back in, for ages. In the meantime, I tried to recover as much deleted stuff from the WD drives, as I could. I connected them with a USB cable to do this.....I still have millions of files to go through!
After this, I re-connected the Caldigit via USB, & all was fine with that until the day before my original posting, above, in July. Upon start-up, that day, it came up as "My Book" (weird, I know....it didn't when it was brand-new), & also reported as empty. Gutted!!
So, as things stand, I have my iTunes collection back (thanks to Data Rescue 3), & it is now on a new usb 8GB Seagate drive. I don't have the available funds for the expensive stuff that you kindly provided the links to, sadly. Thank you, anyway. I am now using the 3 original WD drives with firewire, & the rest with a USB hub.
I've looked at the console logs, but can't find anything for that date, using the 'error' filter.
I never use the sleep function. All drives (except the Caldigit) turn on & off with the mac, usually flawlessly!! The Caldigit is always (& I mean always), ejected correctly before turning it off.
Hope this explains it all.
Cheers,
Woody.