i recently upgraded to Yosemite and after that iphoto stopped working
If you want to continue to use iPhoto, update it to the compatibility update iPhoto 9.6.1, see this User Tip:
Can't Update iPhoto because it is not available in the App Store
i do not have enough disk space to migrate my iphoto library to photo
So how to store photos library on external hard disk ?
When you migrate an iPhoto Library to Photos, the migrated version will be created on the same drive as the iPhoto Library. To create your Photos library on an external drive, copy your iPhoto Library to an external drive and drag it to the Photos icon in the Dock to convert it.
Your external drive needs to be formatted MacOS Extended (Journaled) and it needs to be directly connected. It must not be a backup drive for Time Machine or WD Smartware.
If your external drives have a different format, move any data from the drive you want to use to a different volume, and the format the drive as described here for Aperture:
Format external drives to Mac OS Extended before using with Aperture
Once the drive is correctly formatted you can copy the library simply by drag and drop.