Ther have been a number of threads on this issue, among them

http://social.technet.microsoft.com/Forums/en-US/officeappcompat/thread/7f3f1586-8934-423c-8b29-f19159b49761

http://social.technet.microsoft.com/Forums/en-AU/officeappcompat/thread/9b5776d3-32d9-4fc3-89ec-da07c84fc33b

but none of the suggestions or solutions in these threads or Microsoft technical articles apply.   I think it must be an Access 2010 bug,

This was an Access 2003 database that I saved as a .accdb.  I have been trying to edit a table in Datasheet view.  There is no VBA code involved.  The table has a primary key.

This is an "assignment" table that creates a many-to-many relationship between two other tables, and consequently has two fields, each of which is limited via a Lookup Wizard query to Query representing a selection from the table involved.  In Datasheet view you can then select appropriate values for each field from a dropdown list.

The problem definitely does not come from the Lookup Wizard queries or indeed anything about the table structure.  If I simply use Copy and Paste to duplicate the table, structure and all, the copy is updateable in Datasheet view the way I want it to be.     But of course a number of other queries and reports depend on the original table, and you can't just substitute a copy in.

I think what corrupted the table was an occasion where I accidentally tried to select a duplicate entry for the primary key.  The table was updateable up to that point, but after dismissing the dialog telling me my change would create a duplicate value, I began getting the "recordset is not updateable" message in the status bar.

 

Have you already tried to compact&repair the database? Are the values in the primary key field still unique and valid numbers?

Compact and Repair didn't fix it, and the duplicate table had all the same values and was editable, so the primary key values must be OK.   I don't think the aborted inadvertent duplication was an issue because I did that to the duplicate table and that didn't make it readonly.

Deleting all of the table's relationships didn't fix the problem.

OK, I have a workaround, but it's a really weird one.   I'm 100% convinced it's a bug now.

Despite the datasheet-view recordset not being updateable, I can open the table in design view and change its structure, for example, add a dummy text field.   After that, the table is editable!

Just to make sure, I restored the .accdb from backup and repeated the steps of deleting relationships, compacting and repairing the database, and finally adding the dummy field, closing and reopening the database between steps.   Only the dummy field addition made the table editable again.

 

I've isolated the cause.   I sorted the table by the non-primary-key field (by right-clicking on the column header). After that, the recordset was not updateable and could only be fixed with a structure change (which for some reason restores the default sorting criteria).

Same bug is in Microsoft Access 2007.

I guess you were sorting a lookup field.

I work around the problem by going to table design view -> Properties -> Sort -> change all lookup fields to fields in the table.

For the disadvantages of lookup fields, please refer to http://access.mvps.org/access/lookupfields.htm

Sorry, no, this is not the place to rehash someone's ideology about lookup fields, this is about a bug in Access.

But I will say I will give up lookup fields when you pry my cold dead fingers from them.