Re: Vain bully
Presumably an SSD? There are a number of gotchas with SSDs that can lead to precipitous performance degradation (particularly write speed) due to the SSD running out of space, it might have to load blocks - remember that SSDs use 256kb blocks internally, exposed as 4kb pages - and combine multiple pages into unused parts of currently used blocks. The degradation is called SSD write amplification, and is affected by many factors.
Your hard drive doesn't even have to be "full", it could just be thinking it is running out of space; its not necessarily having 80%+ space used as visible to your OS, if you write and delete a lot of files, then you might start running out of pristine blocks for the SSD to work with, which really can lead to disastrous performance.
The firmware on the device itself can also play a part; there was an infamous firmware from (I think) OCZ, it used a fast algorithm for garbage collection when there were >50% blocks unallocated, but as soon as it went below that it absolutely killed the performance. This gave it great performance when new, but crappy performance in actual usage.
If your drive was quite full of your data (>80%), then getting a newer, larger drive probably was the right answer. Installing a large OS update, which typically deletes a lot of files and writes new ones, could easily have pushed this drive into a state where the performance was just terrible, and either a new drive, or deleting a lot of data, wiping the drive, and restoring the needed data from backup are the options. The genius probably followed the script and upsold you rather than doing the maintenance.
Whether its right or wrong that drives that say "500GB" shouldn't be filled with more than 400GB is a completely different matter - and in fact, some of the "better" brands these days are better simply because they over-provision their drives with more blocks than the others, eg they sell a 100GB drive that actually has 128GB of storage on it. Some drives allow you to configure the amount of over-provisioning to allow for this, eg enterprise drives typically use 28% over-provisioning, where as consumer devices typically do either 0% or 7%.