Replace the size 2032 CMOS battery.  Should be done when buying used and refurb places don't replace them that I know of.  Battery lasts around 5 yrs.

You may have a dead HDD.  Do you have another HDD you can try?  There's no fix for a HDD if it goes physically bad.  I'd try replacing the battery first.  It's a cheap fix and at worst you have a new battery.

This could also be an opportunity to put the OS on an SSD.  SSD's breathe new life into older machines.  Maxing the RAM and installing an SSD speeded up my machine more than putting a quad CPU in it.

Just in case worse does come to worse, did your 760 come with a return policy?

Optiplex 760 Tech Guide 

i bet too you didnt do the DEEP HDD TEST>

most HDD testing programs   , do not test the whole disk, for sure not R/w Test full. or it eats the OS to death.

it only does R./ w on some places on the disk,  and a 10TB disk test is like 10 days  long. (full)

that is why SMART was invented 2001y, just for you everyone,  it tells you EOL< in 1 second flat, IN FACT IT KNOWS IT WAS BAD  before the test. (most times) in fact if turned on W10 (got that?) warns you this as soon

as the HDD flags, EOL it tells you that. (hdd0 fails smart)!!!

the smart runs any time the PC is on, the SMART keeps tract of bad sectors and when the spare sectors RUN DRY the HDD IS NOW TOAST ! as they all do in 5 to 10 years range, used lots.

the linux DISKTEST is  one of the best, and is free, and shows ALL DETAILS> see the orange screen bad here.

what could be  more clear,  IMMINENT ! (self sector repair ends NOW)

old PC

RTC coin cell dead is  #1,  we test our, with a voltmeter, this is first thing to do on all old PCs,

then clean it, if you dont and HS fins/fans packed in lint it overheats, all PCs like that do , when like that.

next is the espa test, it told you the HDD is bad, but you chose to ignore that.

SMART testings and engines:

Imagine a bucket and full of water

you see bottom holes, there,it leaks (smart fails)

but it still holds some water, but you ignore the holes and leaks. why>?

you could load the OS 10x and NO GOOD WILL EVER COME.

if the real test, using real apps, like crystaldiskinfo.com or Ubuntu  linux v16+ disktest shows that

Unrecoverable more than 0 the HDD IS TOAST, no if's nor but's.

HDD can be dead, good or weak ,  dead and weak are trash fodder.