Internal storage is 75 full. Tap to transfer data to SD card. What should I do?

[AC Question]

Internal storage is 75% full. Tap to transfer data to SD card

Sony Xperia Z2
Andriod version 4.4.4 (cannot get anything more recent at the moment )

Notification in notification bar
[Internal storage is 75% full.
Tap to transfer data to SD card]
=> Tap on notification
Pop up window appears and reads
[Transfer data to SD card
Photos 1131 files 1.45GB
Video 0 file, 0.0KB
Music 0 file, 0.0KB
available space on SD card 13.75GB
Cancel // Transfer]
=> Transfer
Almost immediately it comes up with another pop up window reading
[Note!
Transfer interrupted. Check your SD card for possibly transferred data.
OK]
=> OK
New notification in notification bar reads
[Transfer interrupted]

I tried moving the photos across to the SD card manually through the computer and it didn't work, made my computer crash or something (can't remember, it was a while ago). My phone has been telling me for a number of weeks now that the internal storage is full.

What should I do?

 

[B. Diddy]

Re: Internal storage is 75% full. Tap to transfer data to SD card

Welcome to Android Central! The SD card might be faulty, defective, or counterfeit. Try a different card.

I would also suggest moving the photos to your computer hard drive as well, if only as a backup. Mobile storage can fail, and if that happens without any backups, then the photos are lost forever. Connect the phone to the computer via USB and use Windows Explorer to copy the photos to the hard drive--or is that what's causing the crash?