Lost Files transferring iphone to external hard drive

Hello,

I own an iphone 6s and Seagate Expansion External Hard Drive.

I always transfer my photos/vids from iphone to my ext. drive I did NOT back up anything on icloud.

Today I connected them both to my desktop per usual.

I transferred Folder 135 (896 photos/vid) and Folder 136 (144 p/v) to Folder August 2018 on my ext. drive.

Once the transfer was complete, I checked Folder August 2018 to confirm that iphone folders 135 & 136 had transferred successfully, which they did.

Safely removed ext. drive.
Deleted folders 135 & 136 from iphone FROM my desktop, then disconnected my iphone.

I plugged Ext. Hard Drive into my laptop, opened Folder August 2018 which now ONLY contained Folder 135. The Folder 136 was gone.

I returned to my desktop and opened Folder August 2018 which also showed only Folder 135, but this time instead of 896 items there were only 772! I safely disconnected everything. I repeated the process and Folder 135 now has only 127 items!

Does anyone know where these missing files went or how it happened? Also, is it possible to recover them or are they gone forever? :/

Please help,
Jules