comments inline:
You are moving a library from Photos to Photos? Not using iPhoto on the old device and Photos on the new?
If it's iPhoto to Photos, review this post https://discussions.apple.com/docs/DOC-8461
>> the same app on the old laptop (Photos). OS version was 10.12 on it.
Housekeeping
You might be saying the same thing just using different words in your post.
- I believe the Photos library extension should be .photoslibrary, not .libraryfile
>> correct.
- When holding the Option key and opening Photos, it should ask for "choose library," not import.
>> it did.
Your library file may have become corrupt in transit.
Repair the library, https://support.apple.com/en-us/HT204967
>> linux/unix guy (35 years in). used cpio to backup the ENTIRE ~/Pictures folder and brought that across. don't think
>> its corrupt. the app was up and running on the old system however.
Finally, this is just my personal preference, but I almost always use iCloud sync amongst devices. I subscribe to iCloud, but if you don't it's easy enough to purchase the necessary storage for just one month to sync devices. That way I can "follow along" as pictures and videos are moved. Depending on the size of the library it may take longer but I've never had issues that way.
Purchase iCloud space: https://support.apple.com/en-us/HT201318
Turn on sync: Photos > Preferences > iCloud (turn iCloud photos on). Photos > Preferences > General (ensure that both devices are set to system library -- if it is greyed out, that library is the system library)
Wait. iCloud upload sync is throttled to something like 1,000 and hour, 10,000 a day, 25,000 a month. (I don't recall the exact numbers, but transfer could take a day or two).
>> she does have icloud space purchased (200GB)...so this may be the way to go.
>> will try this if migration assistant doesn't offer any help.
