Just ran EtreCheck and it says my hard drive is failing. What should I do?

The 2012 MBP's hard drive is easy to replace - the keyboard is another issue. The replacement isn't what I'd call user friendly, you basically have to tear the whole computer down to get to the keyboard - that's why it is an expensive repair even though the keyboard itself is relatively inexpensive. OTOH it could just be a dirty key so it would be worth your time to do a search on sticky or dirty unibody MBP keyboard and see what can be done.

If the computer is otherwise in good shape and you can get that one key working properly with just some cleaning, replacing the hard drive with an SSD - you may also need to replace the SATA cable - would be worth it. If/when it dies the hard drive or SSD can be removed and put into an enclosure to use as a backup for the new computer. But I wouldn't invest $$$ on a full keyboard replacement on a 2012.

Due to new limitations on the number of characters in a post here, we cannot see all the most useful diagnostics in the Etrecheck report. However, if you have any anti-virus software, adware, or so-called "cleaning/tune-ip" apps installed, get rid of them and rerun Etrecheck. Although the hard-drive diagnostics in the latest Etrecheck are first-rate, we have seen some cases where unneeded and unwanted third-party system hacks can trigger the hard drive failure notice and that notice goes away once you clean off all the scamware.

Another consideration for your model: In the 2009-2012 non-Retina 13-inch MacBook Pros like yours (and mine) the hard drive cable suffers a convoluted routing over sharp edges in the machined metal chassis. After 3-5 years of vibration from normal use and transport, the cable can begin to chaff at the points where it contacts those sharp edges. The symptoms of a worn cable can mimic those of a failing hard drive.

In that model I would, after getting rid of any scamware you installed and finding the computer is still slow, replace the hard drive CABLE first and see if things improve. If not, and you have to install an new drive, you'll start with a fresh cable and no worries about the wear issue.

If you install a new drive, I suggest a solid state drive (SSD) but, regardless of whether you chose an SSD or traditional hard drive, install an SATA III (6GBps) drive. Apple saved money in that model by using a slower SATA II (3GBps) roto-drive and that is what makes those feel "slow." However, the HD connection on the logic board is the faster 6GBps.

When my MBP13 2012 was about a year old I increased the RAM from 4 to 8GB. There was little if any perceived speed difference. Last year I replaced the slow 500GB roto-drive with a 500GB SSD SATA III (6GBps) and the difference in performance is stunning, to put it mildly. The compute is a joy to use now.