Successful backup, now TM says drive is "read-only"

Question

Here's my story:
On Sunday I upgraded my relatively new MacBook Pro (meaning, I haven't had it long enough to install any wonky third party applications) from 10.4 to Leopard. TM was one of my primary motivators for the upgrade. I plugged my brand new 500GB MyBook external FW drive in to my older 400GB MyBook FW drive, and plugged that into the laptop. I'd had the older drive formatted for Mac by the eBay seller and use it to store movies and such. I got the 500 off the shelf at Costco.
My internal hard drive name has a space in it, but in file sharing the computer name does not have any non alpha characters.
I launched TM and pointed it to the new drive as the backup location. It said the drive had to be formatted. I was not surprised. I told it to go ahead.
It finished in a reasonable amount of time, naming the drive "Time Machine Backup" (or something similar, with spaces in the name) and started making the initial backup. I proceded to play a couple hours of World of Warcraft (but that's another forum while it worked. It was still going when I called it a night, but it was making progress backing up about 100GB.
In the morning it was done. No error messages. I accessed the freaky stargate display and saw tons of backed up files and applications. Good so far. I shut the computer down and headed for work where I'm forced to use an HP and Windows.
After booting up last night TM displayed a message (I'm paraphrasing here since I'm at work now): "Backup Failed -- backup disc is read only."
Huh?
I did "Get Info" on the disc and it says I have read/write access. Guests have read only. I was not able to change these permissions (the Get Info window is locked).
I brought up Disc Utility thinking to fix permissions. I can only fix permissions on the internal drive, this option is greyed out for both externals. I verified permissions on the main drive, just to be safe. No issues found.
I changed the FW daisy chain, putting the 500GB drive first so it's directly connected to the computer. No luck, didn't really expect it.
I dragged a file from the main drive over to the backup drive. It copied. I trashed it from the backup drive. It trashed. Definitely read/write.
Key Questions:
Obviously I can read and write to the drive, so why can't TM?
What do I do to fix it?
Secondary Questions:
The system offered to format the drive and I let it. If drive format is the problem here (which seems to be the case with others having problems), why the heck wouldn't the system format it correctly?
If spaces in the drive name are a problem (perhaps they aren't), why would it use a name that has them?

Answer 1

Okay everyone, I think I have found the solution, especially if you are seeing the "Unknown" user in your Get Info window's Sharing & Permissions setting for your external drive.
I had written earlier about my work-around for getting my drobo to work right after getting the "read-only" error from Time Machine. But unfortunately it was a temporary and temperamental fix. There were times then I had to eject the disk and launch Disk Utility to run a repair on the disk which would take more than half an hour on my drobo. But all the while my permissions had this "Unknown" user. Well I decided to "Go UNIX" and see if I could fix this problem once and for all.
I opened up Terminal and typed "*ls -la /Volumes*". This gives me a directory listing of my mounted drive volumes. I noticed that all of my drives, internal, external, USB and FireWire, all had "root" as the user and "admin" as the group, _EXCEPT for my drobo!_ So I forced it to conform by changing it manually by typing "*sudo chown root:admin /Volumes/Drobo\ Backup\ HD/*".
sudo = temporarily enables root mode by asking for your password before executing.
chown = changes ownership to...
root:admin = "root" is the user and "admin" is the group.
*/Volumes/Drobo\ Backup\ HD/* = My drobo drive is named "*Drobo Backup HD*", but UNIX sees spaces as command separators, so you have to use the "\ " to signify spaces within a name.
After doing this over a week ago, _I have not had any other problems_ with using my drobo as a backup drive. It has never come up with the "read-only" error in Time Machine, the Sharing & Permissions setting do not have any "Unknown" users or groups, I don't even need to wait for my drobo before logging in like I did in the past. I just boot my Mac and everything with my drobo and Time Machine just works... No problems!
For me, this seems to be permanently fixed!
But please let me and the rest of us know if you try this solution, and if it works for you.