That happened to me. The iOS 8 upgrade "broke" many of my apps and made the iPad sluggish. So as soon as 8.02 came out, I tried to upgrade, thinking they had fixed whatever was wrong. But while upgrading to 8.02, I got the image of a lightning cable under an iTunes icon on the iPod, and a message "The iPad "name" could not be updated (3014)". Well, that was silly; it's a current model and should be perfectly compatible with any upgrade.
So I did what the picture seemed to suggest - I unplugged my iPad from the computer, then re-plugged it in, and iTunes said I needed to wipe it and restore from backup. OK, progress. I did so last night, and my iPad seems to be working again.
Unfortunately - and this is really a separate issue - it lost most of the information. It didn't just reset the iPad - I told it to restore from backup, and it did restore some things. I think the backup it used (the one made just before I installed 8.02) was corrupt, incorporating whatever problems that were introduced with 8.0.. But there are some things I want to recover.
Unfortunately, the backup I made a week ago (before upgrading to 8.0) I made on an old laptop running Mountain Lion. I was away from home and my normal host computer, and I know iCloud doesn't back up everything. So, not thinking I'd ever have to use the backup - I've never needed to before -I made a complete backup on this old computer before upgrading, just on general principles.
Now I want to restore from the backup on the old laptop. I've copied the files onto a flash drive, but I don't know if the file format of a backup from an old version of iTunes is really compatible with with restoring my iPad using the latest version of iTunes. And, assuming it's compatible, can I restore from the thumb drive, or do I have to put the backup into the (hidden) backups folder on Mavericks? Any help would be appreciated.