How can I unlock iPhoto Library that incorrectly thinks Time Machine busy?

I copied my iPhoto '11 library (310GB) onto an erased internal drive for better speed (getting too many iPhoto crashes when importing with library on a network server, and it takes hours and hours).
But after working a couple times another import crashed, and a Time Machine backup had started. I stopped the backup, but iPhoto launches with this message:
"iPhoto will open as soon as the iPhoto library is available Time Machine might be backing it up."
So I re-enabled Time Machine and let the backup run to completion, figuring stopping it must have left it locked, and a normal completion would unlock it.
No luck.
I'm completely frozen out of this library because I always get this error message, and there's no reason the library should be locked.
How can I unlock it???