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If you have any data on this SD card, try to back up. Then format the SD card from the phone itself and see if it is fixed.
Hi again @Marinos,
How do you mean that you "fixed" it? What exactly did your PC do with the SD-card? Does your SD-card behave the same way in another phone or computer?
Do you have another SD-card you can insert in your phone? That way we can rule out if this is caused by your phone or not.
When did this start happening? Has the phone been bent, dropped or something like that? Has ever a SIM-card been accidentally inserted in the SD-card slot?
@Marinos, in that case this might unfortunately be related to hardware. In order to send your device to an authorized service center I suggest that you contact your Local support for information on how to proceed.
To rule out this being software related you can perform a software repair using Xperia Companion.