Prior years filed with purchased TT disks. Can I sign in and file online this year????

Yes, you can use the TurboTax online editions for tax year 2021.

See this TurboTax support FAQ for how to transfer the 2020 tax data from your computer to the 2021 online return - https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-turbotax-online/00/25953

If you are used to the CD/Download TurboTax software, you might not be happy with TurboTax Online. It lacks many of the features that are in the CD/Download software. The biggest difference is that TurboTax Online does not have forms mode. There is no way to see the actual tax forms except by creating a PDF of the return, and there is no way to make entries directly on the forms. All entries have to be made in the interview. The lack of the forms mode also means that certain other features are not available, such as Data Source and QuickZoom to trace the data flow and see how numbers in the return were calculated. And there is no way to view or print a form that is not required for your tax return.

A TurboTax Online account can only be used for one federal tax return and the associated state return(s). If you need to prepare another return, you have to use a separate account. TurboTax Online saves only the most recent version of your tax return. There is no way to save different versions, or to save a copy of a return with a different name. And all entries and changes are saved immediately and permanently. There is no way to exit without saving.

Only the Self-Employed edition of TurboTax Online supports all forms and handles all tax situations. The lower editions lack certain forms. For example, you can't report investment sales (Schedule D and Form 8949) in Online Deluxe. You need Premier or Self-Employed for that.

TurboTax Online shuts down shortly after the October 15 extended filing deadline. When it comes back up in early to mid December, it's only for the next tax year. So there's a period of a month and a half or two months when you do not have access to your online return at all. After it switches to the next tax year in December, you can only download your tax return from the previous year (the PDF or the .tax20yy file). You can't open it to review the entries or manipulate it in any way. You are strongly advised to download both the PDF and the .tax20yy data file after you file your return and save them on your own computer in case there are any problems accessing the online copies.