"Mike, I want to thank you. Based on your suggestion I think the mounting issue may be fixed. I could not mount my external Firewire (Oxford 911 chipset) drive so I did a RESTART with the FW drive ON. As expected, it booted fine. I then opened the OS X Disk Utility, selected the external drive and ran a Verify and Repair of the drive. It took forever with no errors reported. The good news is I can now dismount the drive and get it to show up again by just turning it ON. It is working perfect again...like it did prior to the 10.5.6 upgrade.

I still attribute the problem to the Apple 10.5.6 update; however Disk Utility seems to have corrected the problem. I hope it keeps on trucking. Thanks for the reminder to try the DU Verify/Repair.
-Richard"

Hopefully it stays that way. (IIRC sometimes when OS X has a problem with a drive mounting it can result in a bad entry in the boot drive's Volumes listing.)
Update - another mail regarding OS X/FW drive problems:

"Hey Mike, about the Firewire mounting issues. I have two WD MyBook drives, both using FW 400, one 500GB and one 1TB Raid 1, using the WD Drive Manager software on a iMac G5 with 10.5.6. We use the Raid 1 (mirror) drive for 'Media' and stream movies to ATV, stream music, keep the iPhoto Library on there, etc.
For about a week the WD manager was always popping up with a message that the drive could not be put to sleep as it was in use. It was odd and annoying, but nothing seemed broken, until a couple days after xmas and my wife asked where all our photos went, all she could see were the ones from Xmas this year.

Sure enough, iPhoto only had the most recent 3 imports. I restarted iPhoto with the OPTION-key pressed down and pointed it to the iPhoto Library on Media and all the old photos were there but not the most recent three imports. Knowing there had to be another iPhoto Library somewhere I looked around via the command line. Here is what I found:

/Volumes/Media -> local directory - ?!
/Volumes/Media 1 -> Raid 1 ext drive
/Volumes/iMac Backup -> 500 GB ext drive

At some point after 10.5.6 - the system created a real directory in /Volumes called Media - and iPhoto created a new Library and iPhoto worked out of there. iTunes however continued to look at the real Media drive (the Raid 1). The WD Drive Manager was very confused and tried to constantly unmount /Volumes/Media.

Even though the Raid drive was listed in /Volumes as 'Media 1' it would still display on the desktop as 'Media'. So, after shutting everything down, I renamed /Volumes/Media to /Volumes/Media_WTF - got the photos out, remounted everything properly, made sure iPhoto and iTunes were pointing to the right place and then got all the photos back together. And the /Volumes folder is now as it should be and the WD Drive Manager has been quiet.
I've been using or managing macs for 15 years now and this was a first for me. But if any readers are seeing weird FW issues, have them check the /Volumes directory for any 'double entries'
Happy New Year, Garth"

For the record, since the night of the 10.5.6 update (I didn't use SU) I've used 3 Oxford bridge board external drives without any problems with mounting, sleep or wake.

• A very old (6 or 7 yrs old) portable case w/Oxford 911 (FW 400) bridge (seagate PATA 120GB HD inside) So old it doesn't have big drive support. Just tested it again - it mounts, opens, sleeps and wakes fine here. (Connected to AL iMac running 10.5.6 w/all updates to date.)
  
  
• Ministack v3 (quad interface - oxford 934 bridge I think) - no problems. (Always connected to the 10.5.6 AL iMac via FW800 - used for manual file backups and TM backups daily - slept/woke dozens of times a day, 7 days a week since Dec. 2007)
  
  
• EliteAL Pro Dual-Drive Quad Interface (Oxford bridge) case - mounted a WD 1TB GP (new 3 platter/32MB cache model) in it for cloning the Hitachi 1TB in the Ministack. (Then swapped the Hitachi out for the WD GP 1TB as it runs much cooler - much better choice for that compact case as I noted in a post here some weeks back)

But as far as 10.5.6, it's been years since I had so many problem mails on an OS X update. (Many used SU but not all.) Knock on wood I've not seen any problems so far after updating (via combo D/L) the night I posted it here.

• Periodic Dual-link display problem (distortion) with new adapter (includes Pix/QT Movie links)
• Mini Displayport to Dual-Link DVI not working?

• Rumors of Large Form iPod Touch To Launch in Fall '09
• MacZot New Years Special: 11 Apps for $59 (MacCleanse, GarageSale, DEVONthink Personal, AppZapper, Xslimmer, Media Rage and more)
• Cocktail utility New Years Eve sale (1/3 of normal price)
• Noise Ninja for Aperture 2.0.5
• Tap Defense 1.2 update (free iPhone/Touch game)
• Aurora Feint the Beginning v1.4 (free iPhone/Touch game)
• OS X Server 10.4 and 10.5: How to remove print jobs from the CUPS queue directory (Apple)
• Apple rumored to be planning new home server device
• New HP MediaSmart home servers automatically back up Macs
• Jobs' health rumors and a Whole Lot of 'Here We Go Again' (cnbc)
• Opinion: For Apple, '08 ends with the Macworld clunker
• Another case maker adds to the iPhone Nano speculation
• Researchers Devise Undetectable Phishing Attack
• Web browser flaw could put e-commerce security at risk
• Microsoft downplays Windows Media Player bug
• Intel Launches Fresh Mobile Quad-Core Chip