>A command as simple as:
>
>Select TABLE_A
>Append from TABLE_B FOR FIELDX=.T.
>
>it appends all records, not only FIELDX=.T.
>FIELDX in TABLE_B is a logical field.
>
>I tried also
>
>Append from TABLE_B FOR TABLE_B.FIELDX=.T.
>
>Same result.
>
>Why things apparently so simple are sometimes so complicated?

According to the Hacker's Guide, in APPEND FROM the FOR clause examines each source record as if it had already been added to the target table. This can cause unexpected results if you're trying to use DELETED(), for example.

I just tried what you're doing with 2 cursors, and it worked as you wanted:

crea cursor test1 (lLog L)
appe && added 4 records, 2 .T., 2 .F.
crea cursor test2 (lLog L)
appe from dbf("test1") for lLog
* Result: 2 records added to Test2, as expected

Do you have any triggers etc. on your target table?