Very slow Time Machine on Drobo 2G


I’m experiencing a very slow Time Machine backup issue on a Drobo 2nd generation.

First of all I must said that I’m using an iMac G5 1,6ghz 17" (2go ram) with OS 10.5.8 connected to the Drobo with FW 800.

This Drobo is set to be a 16TB volume but the real volume space available on the Drobo with 4 disk of 2 TO is almost 3,6 TB. Normally Time Machine will do in this case (wire connection) a Backups.backupdb file and will fill the volume as if was really a 16 TB one because the Drobo appear to be like this.

So, I artificially created on it a sparsebundle (like a Time Capsule backup) with a limited size of 3,5 TO to limit the size of the backup. In this manner I can also, after adding bigger disk in the future, enlarge the size of the sparsebundle if necessary.

The backup started normally quickly and unfortunatly, after copying 5 GO, began to be very very slow. But there is 2,94 TO to backup !

I’ve downloaded also the Time Machine Buddy widget and I found this information (“G5-TimeMachine” is the open sparsebundle, my mac internal HD is “iMac-298” and there is three others HD connected to it : “Musique-S-298” + “Drobo-1” for files and “Drobo-2” for backup) :

Starting standard backup
Disk image /Volumes/Drobo-2 1/G5_0011246fca1c.sparsebundle mounted at: /Volumes/G5-TimeMachine
Backing up to: /Volumes/G5-TimeMachine/Backups.backupdb
Event store UUIDs don’t match for volume: iMac-298
Event store UUIDs don’t match for volume: Musique-S-298
Event store UUIDs don’t match for volume: Drobo-1
No pre-backup thinning needed: 2.99 TB requested (including padding), 3.39 TB available
Waiting for index to be ready (906 > 0)

I see in this webpage http://web.me.com/pondini/Time_Machine/D2.html that “Waiting for index to be ready” can have multiple causes. So I changed cables, port connections, disabled Spotlight, etc. without success. I can’t ask Energy Saver not to spin down the drives (not in my os 10.5.8 preferences, only sleeping option can be check), but I don’t think that is the problem.

Is there a known issue about that ? Any idea ? All I read about slow Time Machine and slow Drobo don’t give me solution.