In the Options | Advanced tab, make sure that you have chosen to open
databases using record-level locking; this might help (but I don't really
know for sure).

What do you mean by SQL-Transaction?  Are you talking here about a JET
Workspace or if you are using SQL-Server as the backend?  Even when you are
writing single records, you should use a transaction if you are using a JET
Workspace as this may help for locking problems.

Otherwise, with multiple processes writing 1,000,000 records and if you are
using JET, then maybe it's time to replace it with SQL-Server as the backend
database. SQL-Server offers a lot more than JET in term of control over
locking and dead-locking mecanisms.

Finally, you don't say how you are making your updates.  With all the
possibilities, it's always hard to guess what someone is doing exactly and
how the various solutions might apply.

With that many records, are you certain you just haven't run up against the
2GB database size limit? Are you regularly doing "Compact and Repair"?
Right-click the file and look at the size.  Is your database split into
front-end on the users' machines and back-end on a server? If I recall
correctly, _adding_ records to a table does not cause locking problems --  
they were clever enough to fix that problem several versions back. Are any
of those log writes _updates_? That's when you are likely to encounter
collisions.

Are you, in fact, writing to a Log Table in the database, or to a Log File
that is on the disk but not in the database?

 Larry Linson
 Microsoft Access MVP

Sylvain, Larry, David - thank you all for your help. I really do appreciate
it.

To answer your questions all in the same place...

Sylvain, by SQL-Transaction, I mean running a SQL string to insert a whole
batch of records together with an INSERT INTO statement. What we are doing
is opening a recordset in vba and adding new records individually using
.Addnew and .Update etc. What we have is a system which, for example, builds
an updateable recordset, and loops through it, record by record, applying
updates according to rules data held in another table. Each update triggers
the addition of a new record in our log table, held in its own Access db on
the network server. There are often several thousand updates per process,
and a handful of people might be running processes at the same time. I know
it would be much easier and faster to execute a SQL statement, but for
various reasons, we can't.

Larry, we are using a common FE/BE configuration, as you say with the FE on
each user's machine. The 2GB size limit is not an issue, and there are NEVER
any updates done to the log table, only additions as described above.

David, although we have had a number of different error messages, with
similar results, the most common one is err.number 3011 (db is read-only) -
once this occurs for one person, no one can write to the BE log db. The only
table in the log db is a single log table with 6 fields. Most of the fields
have indexes on them, and the primary key is a multiple field index.

We are not using transactions (although maybe we should be?) as we are
opening and closing the log recordset every time we add a record.

Hope this helps! Any feedback is greatly appreciated!
Jim