Can't create backup archive

I have Acronis True Image Home 2009 (build 9,608). I have successfully been creating backups for a while, but starting last week, I cannot create one successfully. Initially, the problem was that it would create a backup successfully, but then fail the validation, but now the problem seems to be that I can't even create the backup.

Initially, it would create the backup fine, but when I tried to validate it, I got an error. Also, I couldn't view the contents of the backup. It didn't matter what type of backup I did... I got the same problem when backing up files or the entire system. I am backing up to an external hard drive which still has 70GB of free space. I'm on Windows XP (SP3).

When I validated as part of the backup task, I got an error "E00040011 Cannot open backup archive on the specified device. Either there is no archive on it or the archive is corrupted." If I validated it as a separate validation task, I got an error "Acronis True Image cannot detect volume 1 of "NewDataBackup" archive". If I tried to explore the backup archive, I got prompted with a file browse dialog that says "Please locate the volume number 3".

The log for this event:
1 Information 11/10/2009 8:35:58 AM Operation Full Backup started.
2 Warning! 11/10/2009 8:35:59 AM Priority changed to Low.
3 Information 11/10/2009 8:36:03 AM Analyzing partition 0-0...
4 Information 11/10/2009 8:36:04 AM Analyzing partition C:...
5 Information 11/10/2009 8:36:05 AM Analyzing partition 0-0...
6 Information 11/10/2009 8:36:06 AM Analyzing partition E:...
7 Information 11/10/2009 8:36:11 AM Create Incremental Backup Archive From: Disk 1 To file: "E:\Acronis Backups\Full copy\Full backup.tib" Compression: Normal
8 Information 11/10/2009 8:36:13 AM Pending operation 138 started: "Creating partition image"
9 Information 11/10/2009 8:36:13 AM Locking partition C:...
10 Information 11/10/2009 9:02:12 AM Pending operation 135 started: "Saving partition structure"
11 Information 11/10/2009 9:02:17 AM Validate Backup Archive Location: "E:\Acronis Backups\Full copy\Full backup.tib"
12 Information 11/10/2009 9:02:17 AM Pending operation 3 started: "Validate Backup Archive"
13 Information 11/10/2009 9:11:25 AM File I/O error (0x40011)
function = "OpenFileW",
filename = "\\?\E:\Acronis Backups\Full copy\MyBackup(1)9.tib"
Tag = 0xF35F747B3B21F833
The system cannot find the file specified (0xFFF0)
code = 80070002)
Tag = 0xBD28FDBD64EDB8B9
14 Warning! 11/10/2009 9:11:25 AM Specify location of the volume 9.
Click Browse to specify volume location, Retry to try again, or click Cancel.: Retry//Cancel/
15 Information 11/10/2009 10:47:28 PM Specify location of the volume 9.
Click Browse to specify volume location, Retry to try again, or click Cancel.: Retry//Cancel/: User replied: Cancel
16 Error 11/10/2009 10:47:31 PM Operation with partition "0-0" was terminated.
Details:
Read error (0x70003)
Tag = 0x9365D74A375E415F
File I/O error (0x40011)
function = "OpenFileW",
filename = "\\?\E:\Acronis Backups\Full copy\MyBackup(1)9.tib"
Tag = 0xF35F747B3B21F833
The system cannot find the file specified (0xFFF0)
code = 80070002)
Tag = 0xBD28FDBD64EDB8B9
17 Warning! 11/10/2009 10:47:36 PM Terminated by user.

However, starting yesterday, I can't even create the backup. This happens whether I backup to the external hard drive or the local hard drive. Curiously, if I only backup the Finance files, it's fine. But if I backup the Documents category, I get the following error in the log:

1 Information 11/12/2009 9:57:36 PM Operation Test Data Backup started.
2 Warning! 11/12/2009 9:57:36 PM Priority changed to Low.
3 Information 11/12/2009 9:57:54 PM Locking partition C:...
4 Information 11/12/2009 9:57:57 PM Create Incremental Backup Archive From: Documents To file: "C:\temp\NewDocumentsBackup.tib" Compression: Normal
5 Information 11/12/2009 10:01:56 PM File I/O error (0x40011)
function = "OpenFileW",
filename = "\\?\C:\temp\NewDocumentsBackup.tib"
Tag = 0xF35F747B3B21F833
The system cannot find the file specified (0xFFF0)
code = 80070002)
Tag = 0xBD28FDBD64EDB8B9
6 Error 11/12/2009 10:07:41 PM Cancelled. (0x4000F)
Tag = 0x1D8EAB676A3F6849
7 Error 11/12/2009 10:07:44 PM Operation has completed with errors.

And here's what I get when I try to back up the entire drive:

1 Information 11/12/2009 10:27:27 PM Operation Full Backup started.
2 Warning! 11/12/2009 10:27:27 PM Priority changed to Low.
3 Information 11/12/2009 10:27:33 PM Analyzing partition 0-0...
4 Information 11/12/2009 10:27:35 PM Analyzing partition C:...
5 Information 11/12/2009 10:27:37 PM Analyzing partition 0-0...
6 Information 11/12/2009 10:27:38 PM Analyzing partition E:...
7 Information 11/12/2009 10:27:43 PM Create Incremental Backup Archive From: Disk 1 To file: "E:\Acronis Backups\Full copy\FullBackup2.tib" Compression: Normal
8 Information 11/12/2009 10:27:44 PM Pending operation 138 started: "Creating partition image"
9 Information 11/12/2009 10:27:45 PM Locking partition C:...
10 Information 11/12/2009 10:52:26 PM Pending operation 135 started: "Saving partition structure"
11 Information 11/12/2009 10:52:31 PM Validate Backup Archive Location: "E:\Acronis Backups\Full copy\FullBackup2.tib"
12 Information 11/12/2009 10:52:32 PM Pending operation 3 started: "Validate Backup Archive"
13 Information 11/12/2009 11:03:41 PM File I/O error (0x40011)
function = "OpenFileW",
filename = "\\?\E:\Acronis Backups\Full copy\MyBackup(1)9.tib"
Tag = 0xF35F747B3B21F833
The system cannot find the file specified (0xFFF0)
code = 80070002)
Tag = 0xBD28FDBD64EDB8B9
14 Warning! 11/12/2009 11:03:41 PM Specify location of the volume 9.
Click Browse to specify volume location, Retry to try again, or click Cancel.: Retry//Cancel/
15 Information 11/12/2009 11:09:26 PM Specify location of the volume 9.
Click Browse to specify volume location, Retry to try again, or click Cancel.: Retry//Cancel/: User replied: Cancel
16 Error 11/12/2009 11:09:26 PM Operation with partition "0-0" was terminated.
Details:
Read error (0x70003)
Tag = 0x9365D74A375E415F
File I/O error (0x40011)
function = "OpenFileW",
filename = "\\?\E:\Acronis Backups\Full copy\MyBackup(1)9.tib"
Tag = 0xF35F747B3B21F833
The system cannot find the file specified (0xFFF0)
code = 80070002)
Tag = 0xBD28FDBD64EDB8B9
17 Error 11/12/2009 11:09:28 PM Operation has completed with errors.