Q I waited for Yosemite to stabilise before upgrading, and have now made the move. Most apps seems to run fine, except for Photos. I first accidentally converted a small iPhoto Library, which worked fine, but I couldn’t find a way to add my master library of around 55 GB. When I double-clicked on that library, Photos appeared to be converting it fine until it reached 71%, when it crashed. Is my master library too large to convert?

A It might be, although others have converted much larger (300 GB) libraries without any trouble. It is worth having another go to see if you can coax the conversion through. Start Photos, and it should open with the smaller library which it converted successfully. Then drag your master library and drop it on the Photos app in the Dock.

Photos should ask you if you want to convert and switch to that library. Let it do so, and give it plenty of time. If the master library converts, it will then be made the default Photos library, and you can switch between it and other converted libraries by dragging each onto its app icon in the Dock, as there does not seem to be any better way to do so.

If the library does not convert, it suggests one or both of:

• the original iPhoto Library has a problem, or
• Photos has a remaining bug, in which case you will need to wait for it to be fixed before you can convert that library.

One way to eliminate the former is to ensure that you have the latest release of iPhoto, version 9.6.1, from the App Store. Open the master library using that, and check that everything appears normal.

Apple details the options available to rebuild the library here: first ensure that you have at least one complete backup copy of the library, then start iPhoto up with the Command and Option keys held down until you see the Rebuild iPhoto Library dialog. I suggest that in the first instance you try Repair Database, and in extremis consider the Rebuild Database option.

Comments It was a painfully long time before iPhoto could reliably open and handle such large libraries. Remember that Photos is still version 1, and will need time to sort such issues out.

The only real alternative is Adobe Photoshop Lightroom, although accounts of trying to migrate large iPhoto or Aperture libraries to that are not encouraging, and Lightroom is far from free.

Grammatical note Photos, ominously like Windows, is a noun in the plural which can be singular or plural, just like agenda. Hence:
We need an agenda.
Photos has crashed.
Windows is infected.

It does not form a further plural, such as *Photoses or *Windowses, however entertaining they may sound.

I do wish that vendors would avoid providing this sort of puzzlement for the masses, and ammunition for the peevers.