"Updating from 2011 was terrible. It took me six and a half hours.
I figured out that Quicken was loading the wrong data file".
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Quicken tries to convert the last file you had open before you started the conversion.
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"When I tried the on-line data update, Quicken imported hundreds of transactions for accounts I no longer use."
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How, exactly, would Quicken know you did not use the accounts any more?
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"OK, I can understand why Quicken did these things, but why didn't it give me a way to let me delete downloaded transactions as a group, rather than clicking Edit then Delete for each one?"
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Quicken has never allowed deleting multiple transactions while they are in the "Accept Transactions ... " window - a more difficult proposition than it may appear*.
Assuming the problem accounts were NOT investment accounts, it was not necessary to try to delete the downloaded transactions, individually, while still in the "Accept transactions ..." window.
The way to handle that situation is:
1.) Backup
2.) Make sure you address any/all legitimately new, or match transactions first ... then
3.) Manually enter a dummy transaction (put something easy to identify in its Memo field)
4.) Do an Accept All
5.) Sort the register on Date Entered
6.) Select all the unwanted transactions (starting with the dummy transaction from #2 above - ending with the last transaction in the register) as you would select multiple files in Windows Explorer
7.) Right-click any selected transaction
8.) Click "Delete"
[Once you have done the Accept All, you could alternatively use the Banking > Transaction report (yes, "report") to accomplish the same end, using pretty much the same technique (though you can sort that report on "Date Entered" ... but you can use the Customize dialog to restrict the visible transactions). There is an "Edit" button in the Banking > Transaction report that gives you the option to Delete any transaction(s) "selected" in the report.]
[* For some earlier Quicken versions, you could delete all the transactions in the "Accept transactions ..." window by doing a "Validate", but I believe that was incidental to the Validate (not part of its purpose), and - to my knowledge - the capability no longer exists.]