Problems with Time Machine

[Stuart Goldberg]

I?ve used Time Machine for a couple years with no problem. Recently it stopped deleting the olds backup. Here?s what I?ve been doing: erase external backup disk with disk utility (the only way I know, can?t drag files to trash) Volume Format Mac OS Extended (journaled), turn off TM, delete com.apple.TimeMachine.plist, go to TM prefs and click Change Disk and choose my external HD. The first backup works fine. After that I have problems. I get a message ?Machine Error, Backup volume could not be found?. When I go to preferences and click on the red ?i? by failed, it says, ?Unable to complete backup. Error occurred while creating Backup directory?. Any thoughts? Thanks in advance.

Below are logs generated from Time Machine Buddy.

Starting standard backup
Cookie file is not readable or does not exist at path: /Volumes/Time Machine/.002241202b5c
Volume at path /Volumes/Time Machine does not appear to be the correct backup volume for this computer. (Cookies do not match)
Backup failed with error: 18

Starting standard backup
Backing up to: /Volumes/Time Machine/Backups.backupdb
Event store UUIDs don't match for volume: Macintosh HD
No pre-backup thinning needed: 24.66 GB requested (including padding), 108.87 GB available
Waiting for index to be ready (910 > 0)
Error writing to backup log. NSFileHandleOperationException:*** -[NSConcreteFileHandle writeData:]: Input/output error
Error: (-36) SrcErr:NO Copying /Users to /Volumes/Time Machine/Backups.backupdb/Stuart Goldbergs iMac/2011-11-21-060353.inProgress/8047D603-A193-422B-B765-58016121FF3A/Macintosh HD
Error writing to backup log. NSFileHandleOperationException:*** -[NSConcreteFileHandle writeData:]: Input/output error
Error: (-36) SrcErr:NO Copying /usr to /Volumes/Time Machine/Backups.backupdb/Stuart Goldbergs iMac/2011-11-21-060353.inProgress/8047D603-A193-422B-B765-58016121FF3A/Macintosh HD
Error writing to backup log. NSFileHandleOperationException:*** -[NSConcreteFileHandle writeData:]: Input/output error
Error: (-36) SrcErr:NO Copying /Utilities alias to /Volumes/Time Machine/Backups.backupdb/Stuart Goldbergs iMac/2011-11-21-060353.inProgress/8047D603-A193-422B-B765-58016121FF3A/Macintosh HD
Error writing to backup log. NSFileHandleOperationException:*** -[NSConcreteFileHandle writeData:]: Input/output error
Error: (-8088) SrcErr:NO Copying /var to /Volumes/Time Machine/Backups.backupdb/Stuart Goldbergs iMac/2011-11-21-060353.inProgress/8047D603-A193-422B-B765-58016121FF3A/Macintosh HD
Error writing to backup log. NSFileHandleOperationException:*** -[NSConcreteFileHandle writeData:]: Input/output error
Error: (-36) SrcErr:NO Copying /Volumes to /Volumes/Time Machine/Backups.backupdb/Stuart Goldbergs iMac/2011-11-21-060353.inProgress/8047D603-A193-422B-B765-58016121FF3A/Macintosh HD
Formulating crash report for process backupd[1453]