Hi everyone, can anyone figure out what kind of damage /dev/sda has?
some time ago there was a break of the electric current.
probably because of that event?
How can I try to fix it now and protect myself from future events? (in addition to installing a UPS)
Replace HDD. If there is eve one bad sector, the hdd is failing.
It may be possible to fix the drive so that the bad sectors are "revived" or are flagged as unusable.
However the bad sectors might not be an isolated event. It is likely a sign that the drive is failing in general. Then you can expect more bad sectors soon. You can try to reformat and fix the bad sectors, and then monitor the health of the drive regularly using SMART tools and chkfs.
Typically a new drive has a bunch of unused sectors it can use to map around bad sectors, and can that way continue working as usual. That is perfectly fine. But if the reallocated sector count keep increasing it is a sure sign that the drive is failing.
I would replace the drive ASAP.