Q: Reading "Preparing Library 24%" for the last 4 hours. Is this normal?

Probably normal - how large is your iPhoto library?

What OS and iPhoto/Photos version are you upgrading from?

LN

what is a "lot of photos"? Some people think 400 is a lot - others think 100,000 is a lot

Shat is the status now after some time?

LN

.

Now it is too long - where is the library located?

LN

If by tonight it is still at 24% (which is likely) you need to force quit, reboot and try again

what version of iPhoto do you have?  And under Yosemite were you using iPhoto or Photos?

LN

right

But since you have iPhoto 9.6.1 go through this prep before restarting the migration  --  Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac

and be aware of how the migration works  --  How Photos handles content and metadata from iPhoto and Aperture - Apple Support

and of the differences in the programs  --  Photos 2.0 vs iPhoto 9.6.1:  Features and Capabilities

And of course always have a good, current backup - especially when you are doing upgrades

LN

38 GB library is not a lot of photos.  My 18 GB library has only 5600 photos so scaling up we would expect 38 GB to be about 12-13,000 photos.

You are welcome

Let us know how it goes

LN