[moved to hardware forum]
Regarding deleting the PP cache files - yes, you can delete them, and do so ASAP for your current situation, even if you add additional storage.
I have at least a few suggestions for your current situation:
1) Rotating drives really clog up and will slow down big time when you have more them more than 80% full
2) Cleaning out to get below 80% full on all drives and then de-fragmenting each drive will help a lot if you've never defrag'd (Windows may put parts of a single file in may separate places on a drive and when you need to access that single file, the drive heads are having to more around a lot; defragmentation is the process of putting the various parts of each file close together on the disk; don't degrag your SSD - then do not need this and it can even hurt performance for other SSD specific reasons)
3) You DO NOT want to use USB connected drives for normal operation unless you have to (i.e. laptop users); permanently connected drives are generally faster and more reliable vs. USB.
Possibly you could get some external drive holders and some new larger drives and:
a. put the new drives in the USB holders and copy your files with directory structure to the new drives (Beyond Compare is an excellent utility to pull this off and it can be told to ignore certain file types that you do not need to back up)
b. Switch the drives - put the new large drives in the PC (i.e. 3 or 4 big drives - 2 or 3 TB drives) and move you 1TB drives to the USB holders