The "thread" error is normal, the "double write" could be serious if accompanied another error by: "InnoDB: Warning: purge reached the head of the history list". The ibdata file could bloat to be a gig+ in size. Reply back with the size of the ibdata,

I have a file called ibdata1 - Size 21,504KB

Corruption is not the problem.... try creating a folder c:\simdata, move your data to the new folder then backup... if that fails... try the sample company

I've managed to backup the file, but found the answer here: sagecity.na.sage.com/.../backup-tips-and-tricks.aspx

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Why are my backups increasing exponentially in size every time I create one? I’m also finding from time to time, the program locks up or freezes and the backup isn’t successfully created.(NOTE: This issue occurs for users who are using Simply Accounting 2008 and later)

I usually find the reason for this exponential growth in size is because the backups are being saved in the SAJ folder. Starting in Simply Accounting 2008, the database was changed from Microsoft Access to MySQL. With this change, the database in Simply Accounting is made up of a Sai file and a SAJ folder. Saving the backups in the SAJ folder will cause the exponential growth in size and in some cases cause the program to lock up or freeze. This isn’t a recommended place to save backups as I’ve found in some instances, saving backups here for long periods of time can cause damage to the database itself.

How do you confirm if the backups are being saved in the SAJ folder? Click on “File/Backup” in Simply Accounting’s home screen and double check in the “Location” field that it isn’t being saved in the SAJ folder. If it is, click on the “browse” button and choose a different folder other than the SAJ folder to save the backup to. Once you do this, then try creating a backup. If you find this doesn’t work, click on “File/Properties” in the home screen of Simply Accounting. Take note of the “Company File” (the name of your database) and the “Location”. Close the program, then use Windows Explorer or the My Computer icon (Computer icon in Windows Vista and Windows 7), to browse to the location of your database. Double click the SAJ folder and remove ONLY THE BACKUPS in this folder. Once this is completed, re-open your database and try creating a backup again outside of the SAJ folder.

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The persons who built a backup utility into Sage that could back up into the place it was backing up from, should be soundly thrashed.  Then, they should be given the opportunity in turn, to thrash the QA people who signed off on it.