Disk space filling up, what is safe to delete?

Yes, I am still a bit naive when it comes to certain things, so it's OK to chuckle about my questions

The hard drive on one of my dedicated servers is partitioned with a small section for the important system files and control, and the other portion is obviously mainly for hosting space.

The problem I'm running into is that the smaller partition keeps filling up rapidly and I'm afraid to delete files that I'm unsure are safe.

I've set WHM not to save monthly logs, so I believe it's just dropping the old log files at the start of each month, so I'm pretty sure that's not the issue.

Can anyone tell me what folders/files are safe to delete on a regular basis to avoid running out of disk space?

I'm running:
WHM 9.9.8 cPanel 9.9.8-C159
RedHat 7.3 i686 - WHM X v3.1.0

Any advice/info on what is safe to delete on both partitions of my drive would be a great help, thank you very much in advance!

 

Thanks bking, I'll try to put together a list of stuff that looks to me like it would be relevant in the folders.

For my backup situation, I keep 2 identical hard drives in the server - master & slave. The slave is simply a duplicate of the master. Every morning a cron job copies everything from the master drive over to the slave drive. (not sure exactly how, I paid my upstream to set it up for me) My idea was to have a "hot swappable" situation for the day when the master drive crashes or burns out.