Short answer: Backup your data and replace the drive as soon as possible, it is failing
Long answer: Yes, you should be worried... The drive is failing, it may die as your reading this, or it could go for months or years, but it is failing. 393k sectors is very excessive and the fact it is increasing means the drive is continuing to degrade. Acronis has a knowledge base article explaining what it is, or from Wikipedia's S.M.A.R.T. entry page:
Reallocated Sector Count
Better: lower
Severity: Critical
Count of reallocated sectors. When the hard drive finds a
read/write/verification error, it marks that sector as "reallocated"
and transfers data to a special reserved area (spare area). This
process is also known as remapping, and reallocated sectors are called
"remaps". The raw value normally represents a count of the bad sectors
that have been found and remapped. Thus, the higher the attribute
value, the more sectors the drive has had to reallocate. This allows a
drive with bad sectors to continue operation; however, a drive which
has had any reallocations at all is significantly more likely to fail
in the near future.[3] While primarily used as a metric of the life
expectancy of the drive, this number also affects performance. As the
count of reallocated sectors increases, the read/write speed tends to
become worse because the drive head is forced to seek to the reserved
area whenever a remap is accessed. If sequential access speed is
critical, the remapped sectors can be manually marked as bad blocks in
the file system in order to prevent their use.
Seeing this parameter at a near zero value is normal, provided it does not increase over time, because that is an indication of degrading hardware and potential imminent failure. Regardless of the results of any self-testing or other information, this drive should be replaced as soon as possible to prevent catastrophic data loss.
Personally, if this value in excess of ~20 (not 20k or 200k, but 20) or an increasing value, I implement backup and hardware replacement procedures as soon as possible.
A quick check of a few drives on some computers on our test bench indicate all have their values for Reallocated Sector Count are 0, except one laptop which shows a value of 8, but it's 2 year history indicates only one reallocation event about 15 months ago.
1Reallocated sectors can be an indication of a failing drive, although I have a 5 year old HGST drive that a blip of this over this three years ago and not a thing since. Perhaps you could post the entire SMART diagnostic output? – acejavelin – 2016-05-22T13:06:30.373
The question is - how many do you have left? Certain SMART parameters have a threshold value. The initial value should also be known. So if you started with 100 and you are at 90 and the threshold is 50, you have used 20% of the repair capabilities that the manufacturer considers in the safe range, but only 10% of the total repair capability. I am quite surprised about the numbers though - reallocated sectors on the disk I checked are 0 and occasionnaly one or two; Possibly HGST has a different approach. – le_top – 2016-05-22T21:20:40.390
@acejavelin I have added a screenshot. Let me know if more info is required. – Tanuj Sharma – 2016-05-24T17:47:51.263
FWIW, a HDD of mine recently failed after it had exceeded 448+ reallocated sectors (a steady increase over 6 months). I assume "390k" must be the number of bytes, rather than the number of "sectors"?! (1 sector = 512 or 4096 bytes ...?) – MrWhite – 2016-05-30T01:26:19.247
Well the SMART parameter says "Reallocated sectors" but i really don't know.... – Tanuj Sharma – 2016-06-02T08:21:28.543
BTW, now there are 460k reallocated sectors. – Tanuj Sharma – 2016-06-02T08:22:14.783
https://superuser.com/questions/1218312/pending-sector-issue-does-this-mean-the-hdd-is-failing/1218317#1218317 – SDsolar – 2017-06-12T05:27:19.107