Time Machine fails most of the time on one Mac

Since updating to Lion and the new firmware (RAIDiator 4.1.8 [1.00a043] ) for my ReadyNAS duo one of my Macs, and only one, frequently fails to backup successfully. The error message Time Machine ultimately displays is "The backup disk image "/Volumes/ReadyNAS/Mini.sparsebundle" could not be accessed (error -1)." What is pretty strange to me is that a Time Machine volume is actually mounted when TM starts up. However it isn't on the desktop with the normal Time Machine icon it is just a generic white disk icon. And it also opens up that disk as if it was a folder. I.e. it opens up a window with the Backups.backupdb folder inside of it. Then after thinking for a long time it displays the error I mentioned above. If I manually unmount the TM Backups disk and have TM try again, and do it enough times, it will eventually work (one or more times, before failing again in the same manner).

I've pasted below the Console log output as well as the output of a ls -la on /Volumes and on /Volumes/Time Machine Backups. I found one other post on issues regarding "dissenting pid". That post is http://www.readynas.com/forum/viewtopic.php?f=71&t=56328&p=318596&hilit=dissenting#p318596. I got all excited thinking that would be my problem and solution but it wasn't. If I turn off TM the ls -la shows no entries for the ReadyNAS or Time Machine at all.

I'm completely stumped because the problem although very frequent is intermittent. There is the rare occasion when the backup proceeds normally. In fact in preparing this post the first try failed with all the details listed here. And then the second try succeeded normally. Hope someone can help with this as the machine is a Mini that mostly runs headless so monitoring the TM success is

The Console shows
12/6/11 2:20:50.143 PM com.apple.backupd: Starting standard backup
12/6/11 2:20:51.427 PM com.apple.backupd: Attempting to mount network destination URL: afp://ReadyNAS@NetgearDuo.local/ReadyNAS
12/6/11 2:20:58.024 PM com.apple.backupd: Mounted network destination at mountpoint: /Volumes/ReadyNAS using URL: afp://ReadyNAS@NetgearDuo.local/ReadyNAS
12/6/11 2:21:39.975 PM com.apple.backupd: Failed to eject volume /Volumes/ReadyNAS (FSVolumeRefNum: -105; status: -47; dissenting pid: 0)
12/6/11 2:21:39.975 PM com.apple.backupd: Waiting 60 seconds and trying again.
12/6/11 2:22:40.031 PM com.apple.backupd: Network destination already mounted at: /Volumes/ReadyNAS
12/6/11 2:23:18.365 PM com.apple.backupd: Failed to eject volume /Volumes/ReadyNAS (FSVolumeRefNum: -105; status: -47; dissenting pid: 0)
12/6/11 2:23:18.366 PM com.apple.backupd: Waiting 60 seconds and trying again.
12/6/11 2:24:18.417 PM com.apple.backupd: Network destination already mounted at: /Volumes/ReadyNAS
12/6/11 2:24:56.521 PM com.apple.backupd: Failed to eject volume /Volumes/ReadyNAS (FSVolumeRefNum: -105; status: -47; dissenting pid: 0)
12/6/11 2:24:56.521 PM com.apple.backupd: Giving up after 3 retries.
12/6/11 2:25:06.531 PM com.apple.backupd: Backup failed with error: 21

ls -la output
Mini:~ ronaldg$ ls -la /Volumes
total 8
drwxrwxrwt@ 4 root admin 136 Dec 6 14:14 .
drwxr-xr-x 34 root wheel 1224 Dec 6 14:15 ..
drwxrwx---+ 25 ronaldg staff 918 Oct 13 09:51 Funster
lrwxr-xr-x 1 root admin 1 Dec 6 14:14 Macintosh HD -> /
Mini:~ ronaldg$ ls -la /Volumes
total 8
drwxrwxrwt@ 6 root admin 204 Dec 6 14:21 .
drwxr-xr-x 34 root wheel 1224 Dec 6 14:15 ..
drwxrwx---+ 25 ronaldg staff 918 Oct 13 09:51 Funster
lrwxr-xr-x 1 root admin 1 Dec 6 14:14 Macintosh HD -> /
drwx------ 9 root wheel 264 Dec 6 14:21 ReadyNAS
drwxr-xr-x 6 ronaldg staff 340 Sep 30 2010 Time Machine Backups
Mini:~ ronaldg$ ls -la /Volumes/Time*
total 0
drwxr-xr-x 6 ronaldg staff 340 Sep 30 2010 .
drwxrwxrwt@ 6 root admin 204 Dec 6 14:21 ..
drwx------ 4 root staff 170 Jul 23 13:37 .Spotlight-V100
d-wx-wx-wt 2 ronaldg staff 68 Jul 23 14:33 .Trashes
drwx------ 2 ronaldg staff 67762 Dec 6 13:40 .fseventsd
drwxr-xr-x+ 5 root staff 170 Jul 23 14:02 Backups.backupdb