SQL Admin question--Transaction log problem?

[Winla]

A SQL7 DB has following transaction log setting

Space allocated 11 MB
Maxmium size 18MB

Used 2.44MB
Free 10.24MB

Database option: Truncate log on checkpoint (checked)


But when I tried to do a modification of a table with 14K records (eg. add one more column), I got an error msg:


'MLetter' table
- Unable to modify table.
ODBC error: [Microsoft][ODBC SQL Server Driver][SQL Server]The log file for database 'MDB' is full. Back up the transaction log for the database to free up some log space.


Since I checked the option 'Truncate log on checkpoint', the server is supposed to clean log file automatically.
What could be wrong?

[JHausmann]

Were other people using the database when you attempted this? The log file is only cleared to the point of the oldest open transaction.

[Winla]

I am the only user for this database now.
What should I check at this moment? Any idea?

[JHausmann]

You might try opening the database exclusively and making the change...

OR

Increasing your log files by a couple of hundred percents...