This was a 6 hour nightmare for me. Here's what I finally had to do:
1. "Print" my returns to a PDF file. (I had to pay for TurboTax before it would let me do this -- including an extra $50 for State. You either pay or delete. Springing this on us on the "File" step borders on criminal, IMHO.)
2. Search my 1099-M (actually Form 8949) for any entry with a blank cost basis
3. Return to TurboTax (NOT where I left off -- it just hangs!), and revisit Federal Income.
4. Find the entries that had the missing basis, click Edit, and fill in the missing info.*
Note 1: TurboTax lists the securities alphabetically. I did not see a way to sort or search, so had to navigate through the 48 pages by clicking ">" or "<". Each of these moves you forward/backward by only one page, and for me, each page took about a minute to load. So if your entries are near the middle, expect a long and tedious process navigating to your entry. After each edit, you will be kicked back to page one, and will have to start the process all over.
It was a horrible experience, but at least I was able to get my taxes filed before the deadline. Needless to say, I will never use TurboTax online again.
*In one of them my broker had specified an "(e)" for "estimated". Apparently TurboTax does not import these. The other two were "cash in lieu". For those I entered a basis of 0.
