I have 3 1TB drives in my array, one of them is getting full, the other 2 are pretty much empty. When Drive 1 gets full does it start using the next one?

Depends on the allocation method you've set for each of your shares:

 

This setting determines how unRAID OS will choose which disk to use when creating a new file or directory:

High-water Choose the lowest numbered disk with free space still above the current high water mark. The high water mark is initialized with the size of the largest data disk divided by 2. If no disk has free space above the current high water mark, divide the high water mark by 2 and choose again.

The goal of High-water is to write as much data as possible to each disk (in order to minimize how often disks need to be spun up), while at the same time, try to keep the same amount of free space on each disk (in order to distribute data evenly across the array).

Fill-up Choose the lowest numbered disk that still has free space above the current Minimum free space setting.

Most-free Choose the disk that currently has the most free space.

Thanks DZMM, That explains it nicely. What setting do most folks use?

13 minutes ago, KellyVB said:

Thanks DZMM, That explains it nicely. What setting do most folks use?

Cut and paste job from the unraid inline help!

 

High water for the reasons given.  I also use share split levels and per share drive inclusion settings to try and further minimise drive spin-ups