Video becomes laggy when I transfer it to my pc and play it.

i would try connecting the the ixpand to a computer and backing up any data that may be on it. format the ixpand and use exFAT as the file system. after that copy a known good video from the computer to the ixpand and play it from the ixpand and see if there are any issues with the playback. if there not then try copying videos from the phone. If there are still issues the drive may need to be replaced.