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I got my biometrics appointment letter next day after I got my receipt notice, which I got 4 days after I efiled my I-90 on September, 18.
I wrote that my previous card was "lost, stolen, or destroyed" because that is what the Texas Service Center instructed me to do in their letter after my expired green card was withheld as some kind of evidence on the US border. One other thing is that I entered some wrong information in the I-90 by accident, the mistake was not really serious, but I expected a delay due to this error. And still another issue, I have 2 valid greencard numbers at the moment, I don't know why.
So I feel really lucky that I got my biometrics appointment letter so quickly.
What is so confusing is that there are I-90 timelines with MSC such as:
Date filed----------Date approved--------Card sent
June 14, 2006------June 28, 2006--------July 3, 2006
June 14, 2006------NOT APPROVED YET---NOT SENT
June 14, 2006------September 24, 2006---October 4, 2006
How many months? I don't know. Something like 6 or more, probably.
From what you told me about your biometrics appointment scheduling, I suspect that the reason for such irregularities is within the USCIS.