AnandTech took an in-depth look at the trends in the hard disk drive market. Shipments of HDDs have dropped significantly over the last two years, last quarter they fell to 99.8 million units, down from 138 million in Q1 2014 and 163 million in Q1 2010. The prime reason for this is the weak PC market as well as the rise of the solid state disk.
You can read the full article over here, it also contains nice charts about vendor marketshare, average HDD capacity, average pricing, etc.
Sales of PCs in general (and hard drives in particular) are seasonally not strong in the first quarter of the year, which is why it is not surprising that they declined to around 100 million units from 115 million units in Q4 2015. What is alarming is that despite this seasonal change, Q1 2015 shipments of HDDs were higher than sales of hard drives in each of the remaining quarters last year. If this year follows the same negative pattern, then HDD shipments will be below 100 million units in the second quarter and will remain below that level in the second half of the year.