Copy how?

Following the mozilla thunderbird Support transfer instructions, Help, Troubleshooting, Profile Folder, Up three levels, Close thunderbird, Right Click on Thunderbird folder and select COPY, Paste to transfer device, 128 GB flash drive, all files successfully copied until the LAST ONE, Inbox then the error message, Ive followed the procedure twice with the same result. However, when I first set up thunderbird on my Lenovo laptop, it uploaded all inbox emails from Comcast, it tool a long time to upload and then a long time to digest but then I had all my old emails on my laptop, would it be easier to just start up Thunderbird on my new laptop and connect to Comcast and let the files all upload again? If this is so fine, however how do I get Thunderbird after the inbox is uploaded, to impor the sent folder from Concast? Thanks.

Is this an IMAP account? If so, there are other options.

Well you tell me, I was using Outlook 2013 and Comcast forced me to use something else so I am using Thunderbird. I think Outlook is an IMAP but not sure. Again why does the Profile copy procedure fail? Can I just start over and upload all from Comcast onto my new laptop, Im wasting more time trying to solve a procedural failure that it would take to start over. Thanks in advance for your input!

Look at your account's server settings pane. Does it say IMAP or POP? Only you can know that. If it is IMAP, there is no need to copy the message files, as the account will automatically repopulate the folders.