updating/moving photos library. Get a error "Photos in use"

So you tried to copy Photos library from an external disk to the internal by copying it via the Finder, right?

Sometimes that fails which is frustrating especially in the very end of a lengthy copy process. I do not know the exact cause but here are some possible causes and fixes for that:

• Sometimes Photos might be busy updating the current library. A workaround might be to create a new dummy library and keep it open in the background while copying the real library. Or maybe do the copy in safe mode (I have not tried that).

• Sometimes Photos automatically creates a new library to ~/Pictures, and trying to copy another same-name library to that folder might cause a clash if that happens during the copy process (I believe I was recently bitten by that).

• iCloud Drive has an option to put Documents & Desktop folders to the iCloud Drive (I have never used that option). Copying a large item like Photos library might take a long while because the files must be uploaded to the cloud via a potentially slow or flaky Internet connection. If that is the case, it is better to copy to some other home folder instead. And besides, storing items like Photos or Final Cut Pro libraries in cloud services is not supported anyway.

• If all that is taken into account and the copy still fails, I'd use Carbon Copy Cloner for the copy because it is more robust than Finder when copying large amounts of files and tiny database files.

BTW make sure the external drive is formatted as APFS or Mac OS Extended (GUID) case insensitive because non-Mac formats like exFAT, cloud or network shares or Time Machine drives are not supported and might corrupt the library.

Adding to what Matti Haveri said, Photos cannot work from a networked drive like iCloud or NAS or OneDrive, etc. The drive must have a physical cable connecting to the Mac. And, as Matti said, the drive drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

If this drive is in a an incompatible format, stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

Let us know what you find…